[compositorworker] compositor proxy mutation updates underlying layers

The CL makes it so that changes to compositor proxies in  compositor worker to
actually mutate the underlying layers.

This is achieved by passing a reference to active layer tree to
LayerTreeMutator. This reference encapsulated by  CompositorMutableStateProvider
which is responsible for handing out  individual CompositorMutableState for each
proxy during rAF mutation.

Any mutation on the proxy updates the mutable states which in turn is reflected
on the underlying layers (main, scroll).

major changes:
- pass active layer tree impl to mutator
- set mutable state for compositor proxy during rAF
- add 'initialized' attribute to compositor worker to signal its activation
- add layout tests covering visual updates from worker
- update most of compositor worker layout tests to use a
common pattern

TEST=virtual/threaded/fast/compositorworker
BUG=430155
CQ_INCLUDE_TRYBOTS=tryserver.blink:linux_blink_rel

Review-Url: https://codereview.chromium.org/2041193005
Cr-Commit-Position: refs/heads/master@{#400082}
34 files changed