GitLab

As part of this, starts allocating a transform node to represent direct
compositing of will-change:transform elements and their stacking children.

BUG=668342
CQ_INCLUDE_TRYBOTS=master.tryserver.chromium.linux:linux_layout_tests_slimming_paint_v2

Review-Url: https://codereview.chromium.org/2571043002
Cr-Commit-Position: refs/heads/master@{#438771}