Revert of Timeline: extract and rename TracingModelLoader into a file…

… of its own (patchset #4 id:60001 of https://codereview.chromium.org/1739193003/ )

Reason for revert:
Possible cause behind failure in webkit-tests on WebKit Mac10.11 and WebKit Mac10.11 (retina).

Original issue's description:
> Timeline: extract and rename TracingModelLoader into a file of its own
>
> This just extracts TimelineLoader with the aim of later  refactoring
> it to untangle timeline lifecycle events from TimelineModel.
>
> Committed: https://crrev.com/b994e8b0a8af48aca6aa2182b5e8157b4873c0e9
> Cr-Commit-Position: refs/heads/master@{#378109}

[email protected],[email protected]
# Not skipping CQ checks because original CL landed more than 1 days ago.

Review URL: https://codereview.chromium.org/1752453002

Cr-Commit-Position: refs/heads/master@{#378291}

devtools.gypi

@@ -683,7 +683,6 @@
             'front_end/timeline/TimelineIRModel.js',
             'front_end/timeline/TimelineJSProfile.js',
             'front_end/timeline/TimelineLayersView.js',
-            'front_end/timeline/TimelineLoader.js',
             'front_end/timeline/TimelineModel.js',
             'front_end/timeline/TimelinePaintProfilerView.js',
             'front_end/timeline/TimelinePanel.js',