cc: Remove MutatorHost methods from cc::Layer

These methods are only valid when the Layer is in a Layer tree attached
to a LayerTreeHost, making them difficult to use. Also, they don't use
any private stuff from Layer so they don't need to be part of its API.

Instead have code that knows about the LayerTreeHost get the
MutatorHost off that. PropertyTrees code could also but doesn't have
the LayerTreeHost/LayerTreeImpl passed in today.

HasTickingAnimationForTesting() was being tested to return false when
the Layer is not attached to a LayerTreeHost, which is testing
test-only code. Remove those cases. Instead have tests query the
MutatorHost directly.

R=weiliangc@chromium.org

Cq-Include-Trybots: luci.chromium.try:android_optional_gpu_tests_rel;luci.chromium.try:linux_layout_tests_slimming_paint_v2;master.tryserver.blink:linux_trusty_blink_rel
Change-Id: Id384e3cef22d62c65e4c63411bdbfd603993f5ce
Reviewed-on: https://chromium-review.googlesource.com/1101416


Commit-Queue: danakj <danakj@chromium.org>
Reviewed-by: Chris Harrelson <chrishtr@chromium.org>
Reviewed-by: weiliangc <weiliangc@chromium.org>
Cr-Commit-Position: refs/heads/master@{#568126}