Add copyright headers in a few files where they were missing.
Trim trailing blank comment line where present, for consistency with
other engine code.
Use the standard libtxt copyright header in one file where it differed
(extra (C) and comma compared to other files in libtxt).
This also amends tools/const_finder/test/const_finder_test.dart to look
for a const an additional four lines down to account for the copyright
header added to the test fixture.
This reverts commit f1f19bba8f0089490962316867bd222727510ac5.
Now we shouldn't break the engine build as https://chromium-review.googlesource.com/c/chromium/tools/build/+/1480746 is landed. The golden test is disabled by default and we'll enable it later in our recipe and test it in presubmit tests.
This breaks the Linux build with:
```
[ RUN ] PerformanceOverlayLayer.Gold
../../flutter/flow/layers/performance_overlay_layer_unittests.cc:70: Failure
Value of: golden_data != nullptr
Actual: false
Expected: true
Golden file not found: "/b/s/w/ir/k/src/flutter/testing/resources"/performance_overlay_gold.png.
Please either set --golden-dir, or make sure that the unit test is run from the right directory (e.g., flutter/engine/src).
```
This reverts commit 3f0ce769e296b977cd0bcd8f4f7390f207ef50a8.
Reverting this broke the flow tests for the performance overlay:
```
../../flutter/flow/layers/performance_overlay_layer_unittests.cc:70: Failure
Value of: golden_data != nullptr
Actual: false
Expected: true
```
This reverts commit 8427d73c8340c9e51492689ee66261a4a72cac9e.
* Revert "Revert "Add mock capability to PerformanceOverlayLayer (#7537)" (#7765)"
This reverts commit 693645e4645001ae460d5794fa6f5c7066677955.
* Add command line args for golden dir and font file
