GitLab

This is a CC-side fix which can be useful for all CC clients.

Blink side: We will need to fix the race in CompositedLayerMapping destruction vs. CompositorAnimationPlayer detachment separately.

BUG=615471
CQ_INCLUDE_TRYBOTS=tryserver.blink:linux_blink_rel

Review-Url: https://codereview.chromium.org/2042713002
Cr-Commit-Position: refs/heads/master@{#398209}