Remove blink web test dependency on --enable-slimming-paint-v2

This patch switches web tests (including flag-specific expectations and
virtual test suites) to use --enable-blink-features=CompositeAfterPaint
instead of --enable-slimming-paint-v2. This will let us remove the
--enable-slimming-paint-v2 command line flag in the future.

To run web tests (formerly layout tests) with CompositeAfterPaint, use:
[run_web_tests] --additional-driver-flag=--enable-blink-features=CompositeAfterPaint

To run the CompositeAfterPaint virtual test suite, use:
[run_web_tests] virtual/composite-after-paint


This patch is hitting a bug in the "without patch" step which causes the
CQ to reject the patch (see: https://crbug.com/912178) so I am landing
without the CQ. Sheriffs should revert this patch if it causes problems.
NOTRY=true

Bug: 909899
Change-Id: Id0b30cf0ea88b3741ce7135737805862da96d891
Reviewed-on: https://chromium-review.googlesource.com/c/1357665
Commit-Queue: Philip Rogers <pdr@chromium.org>
Reviewed-by: Fady Samuel <fsamuel@chromium.org>
Reviewed-by: Xianzhu Wang <wangxianzhu@chromium.org>
Reviewed-by: John Budorick <jbudorick@chromium.org>
Cr-Commit-Position: refs/heads/master@{#614006}
1271 files changed