Enable compositing for opaque scrolling content on low DPI screens

If an overflow scroll element has an opaque background color we
will consider it for promotion to a composited layer, regardless
of the LCD text vs compositing reference. A runtime flag must be
enabled on the command line to test and use this feature. It will
be removed after we verify stability and fix related bugs.

R=flackr, chrishtr
BUG=381840

Review-Url: https://codereview.chromium.org/1826013002
Cr-Commit-Position: refs/heads/master@{#411037}
6 files changed