Add deprecation notices to the headers.

Author: jonsimantov

gma/src/include/firebase/gma.h

@@ -61,6 +61,9 @@ class Future {
 ///
 /// The GMA API allows you to load and display mobile ads using the Google
 /// Mobile Ads SDK. Each ad format has its own header file.
+///
+/// @deprecated The Google Mobile Ads C++ SDK is now deprecated. Please see
+/// https://developers.google.com/admob/cpp/sdk for more information.
 namespace gma {
 
 /// Initializes Google Mobile Ads (GMA) via Firebase.

gma/src/include/firebase/gma/ad_view.h

@@ -63,6 +63,9 @@ struct BoundingBox;
 ///   ad_view->LoadAd(your_ad_request);
 /// }
 /// @endcode
+///
+/// @deprecated The Google Mobile Ads C++ SDK is now deprecated. Please see
+/// https://developers.google.com/admob/cpp/sdk for more information.
 class AdView {
  public:
   /// The possible screen positions for a @ref AdView, configured via
@@ -86,20 +89,20 @@ class AdView {
 
   /// Creates an uninitialized @ref AdView object.
   /// @ref Initialize must be called before the object is used.
-  AdView();
+  FIREBASE_DEPRECATED AdView();
 
   ~AdView();
 
   /// Initializes the @ref AdView object.
   /// @param[in] parent The platform-specific UI element that will host the ad.
   /// @param[in] ad_unit_id The ad unit ID to use when requesting ads.
   /// @param[in] size The desired ad size for the ad.
-  Future<void> Initialize(AdParent parent, const char* ad_unit_id,
+  FIREBASE_DEPRECATED Future<void> Initialize(AdParent parent, const char* ad_unit_id,
                           const AdSize& size);
 
   /// Returns a @ref Future that has the status of the last call to
   /// @ref Initialize.
-  Future<void> InitializeLastResult() const;
+  FIREBASE_DEPRECATED Future<void> InitializeLastResult() const;
 
   /// Begins an asynchronous request for an ad. If successful, the ad will
   /// automatically be displayed in the AdView.

gma/src/include/firebase/gma/interstitial_ad.h

@@ -69,21 +69,24 @@ class InterstitialAdInternal;
 ///   interstitial->Show();
 /// }
 /// @endcode
+///
+/// @deprecated The Google Mobile Ads C++ SDK is now deprecated. Please see
+/// https://developers.google.com/admob/cpp/sdk for more information.
 class InterstitialAd {
  public:
   /// Creates an uninitialized @ref InterstitialAd object.
   /// @ref Initialize must be called before the object is used.
-  InterstitialAd();
+  FIREBASE_DEPRECATED InterstitialAd();
 
   ~InterstitialAd();
 
   /// Initialize the @ref InterstitialAd object.
   /// @param[in] parent The platform-specific UI element that will host the ad.
-  Future<void> Initialize(AdParent parent);
+  FIREBASE_DEPRECATED Future<void> Initialize(AdParent parent);
 
   /// Returns a @ref Future containing the status of the last call to
   /// @ref Initialize.
-  Future<void> InitializeLastResult() const;
+  FIREBASE_DEPRECATED Future<void> InitializeLastResult() const;
 
   /// Begins an asynchronous request for an ad.
   ///

gma/src/include/firebase/gma/rewarded_ad.h

@@ -71,6 +71,9 @@ class RewardedAdInternal;
 ///   rewarded->Show(&my_user_earned_reward_listener);
 /// }
 /// @endcode
+///
+/// @deprecated The Google Mobile Ads C++ SDK is now deprecated. Please see
+/// https://developers.google.com/admob/cpp/sdk for more information.
 class RewardedAd {
  public:
   /// Options for RewardedAd server-side verification callbacks. Set options on
@@ -86,17 +89,17 @@ class RewardedAd {
 
   /// Creates an uninitialized @ref RewardedAd object.
   /// @ref Initialize must be called before the object is used.
-  RewardedAd();
+  FIREBASE_DEPRECATED RewardedAd();
 
   ~RewardedAd();
 
   /// Initialize the @ref RewardedAd object.
   /// @param[in] parent The platform-specific UI element that will host the ad.
-  Future<void> Initialize(AdParent parent);
+  FIREBASE_DEPRECATED Future<void> Initialize(AdParent parent);
 
   /// Returns a @ref Future containing the status of the last call to
   /// @ref Initialize.
-  Future<void> InitializeLastResult() const;
+  FIREBASE_DEPRECATED Future<void> InitializeLastResult() const;
 
   /// Begins an asynchronous request for an ad.
   ///