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This patch adds a feature flag for respecting the MacOS subpixel
anti-aliasing setting (called "LCD text" in chromium). MacOS 10.14
(Mojave) disabled LCD text by default and, by respecting this setting,
scrolling performance can be improved. This is because composited
scrolling no longer needs to be avoided for the purposes of maintaining
LCD text, which is lost in the common case where the scrolling contents
layer is not fully opaque (see [1] for details of this).

The MacOS setting for LCD text can be controlled with:
defaults write -g CGFontRenderingFontSmoothingDisabled -bool true
("true" is default, "false" overrides this). This patch uses a private
API, CGFontRenderingGetFontSmoothingDisabled, which is used by Safari
(see: https://trac.webkit.org/changeset/239306/webkit).

Enabling this feature will expose the differences of composited
scrolling which include minor pixel differences, less-minor stacking
differences (the fundamental compositing bug: crbug.com/370604), as well
as a few bugs. These differences are already present on all devices with
device pixel ratios >= 1.5 (50% of MacOS), as well as Android and
ChromeOS platforms which force-enable prefer-compositing-to-lcd-text.
The web_test changes can be seen in the Mac10.14 trybot results at
https://crrev.com/c/2207413/1. These have been triaged and
non-rebaseline differences have been broken out as bugs blocking
crbug.com/1079418. This patch does not enable the feature by default but
a followup will, which will include these baseline updates.

We have UMA data from the PreferCompositingToLCDText experiment that
should match the effect of this feature for users that have not enabled
LCD text via the command line:
Event.Latency.ScrollUpdate.Wheel.TimeToScrollUpdateSwapBegin4
  95th: -5%, 99th: -12%
EventLatency.GestureScrollUpdate.Wheel.TotalLatency
  95th: -6%, 99th: -14%
https://uma.googleplex.com/p/chrome/variations/?sid=a8cb2e0aa29593ad35baa33a2bee3f97

[1] Excellent explanation of why compositing forces greyscale AA:
https://gankra.github.io/blah/text-hates-you/#subpixel-aa-isnt-composable

Bug: 1079418
Change-Id: Ieca837912ec19f7a7b4378b989d86a4242108668
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2207413Reviewed-by: Elly Fong-Jones <ellyjones@chromium.org>
Reviewed-by: Avi Drissman <avi@chromium.org>
Commit-Queue: Philip Rogers <pdr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#771858}