[rust png] Stall the test HTTP request at a slightly later offset.

After this CL, the HTTP request for a PNG image will stall for 2 seconds
at a slightly later offset than before.  Before this CL, it would stall
at the beginning of the next frame's metadata chunk (`fcTL`).  After
this CL it will stall at the begining of the next frame's data chunk
(`fdAT`).

See the bug for more details on why this change has been picked as a way
forward.

Bug: 393205780
Change-Id: Ic4d3ed45f43815c65320a41bb6b9d92b7368640e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6227950
Commit-Queue: Łukasz Anforowicz <[email protected]>
Reviewed-by: Michal Mocny <[email protected]>
Cr-Commit-Position: refs/heads/main@{#1416219}

largest-contentful-paint/animated/observe-animated-image.tentative.html

@@ -13,15 +13,29 @@
       assert_implements(window.LargestContentfulPaint,
                         "LargestContentfulPaint is not implemented");
       const beforeLoad = performance.now();
-      // 262 is the size of the animated PNG up until the first frame,
-      // including the chunk that starts the second frame (indicating that
-      // the first frame data is done).
-      // The trickle pipe delays the response after the first frame by 1 second.
+
+      // Offsets inside `anim-gr.png`:
+      //   IHDR: 8..33
+      //   acTL: 33..53
+      //   fcTL (1st frame): 53..91
+      //   IDAT (1st frame): 91..254
+      //   fcTL (2nd frame): 254..292
+      //      fcTL payload starts at offset 262
+      //   fdAT (2nd frame): 292..448
+      //      fdAT payload starts at offset 304
+      //
+      // The HTTP request below will stall for 2 seconds after all pixels of
+      // the 1st frame are available, but before any 2nd frame pixels are
+      // available.  The test will verify that LargestContentfulPaint fires
+      // "immediately", without waiting for the rest of the bytes.
+      //
+      // See https://crbug.com/393205780 why this offset (and not an earlier
+      // one) is used in the test.
       const url = window.location.origin +
-                  `/images/anim-gr.png?pipe=trickle(262:d${delay_pipe_value})`;
+                  `/images/anim-gr.png?pipe=trickle(304:d${delay_pipe_value})`;
+
       const entry = await load_and_observe(url);
-      // anim-gr.png is 100 by 50.
-      const size = 100 * 50;
+      const size = 100 * 50;  // `anim-gr.png` is 100 by 50.
       checkImage(entry, url, 'image_id', size, beforeLoad, ["animated"]);
     }, "Same origin animated image is observable and has a first frame.");
   </script>