Promote opaque fixed position elements.

BUG=626200
TEST=CompositedLayerMappingTest.PromoteOpaqueFixedPosition
CQ_INCLUDE_TRYBOTS=master.tryserver.chromium.linux:linux_layout_tests_slimming_paint_v2

Review-Url: https://codereview.chromium.org/2318243002
Cr-Commit-Position: refs/heads/master@{#421986}
5 files changed