Cherry-pick WebKit change r77191 to pick up a rendering optimization

The primary motivation for taking this change is to allow r100677 to be
cherry-picked cleanly. It should introduce no functional change.

See http://trac.webkit.org/changeset/77191

Bug: 6079158
Change-Id: I8dd8ebad7f0196452fd16095bdbcd26346cebc09
3 files changed