cc: Glue LTHI and Scheduler changes for checker-imaging.

This change glues together the scheduling and tile management changes
for checker-imaging via ProxyImpl and SingleThreadProxy for enabling
checker-imaging. And adds integration LayerTreeTests, including a
scroll test to ensure that an impl-side pending tree does not affect
SyncedProperty state synchronization.

BUG=686267, 691041
CQ_INCLUDE_TRYBOTS=master.tryserver.blink:linux_trusty_blink_rel

Review-Url: https://codereview.chromium.org/2717553005
Cr-Commit-Position: refs/heads/master@{#453508}

• cc/BUILD.gn[diff]
• cc/playback/raster_source.cc[diff]
• cc/scheduler/scheduler_state_machine.cc[diff]
• cc/test/layer_tree_test.cc[diff]
• cc/test/layer_tree_test.h[diff]
• cc/test/test_hooks.h[diff]
• cc/trees/layer_tree_host.cc[diff]
• cc/trees/layer_tree_host.h[diff]
• cc/trees/layer_tree_host_impl.cc[diff]
• cc/trees/layer_tree_host_impl.h[diff]
• cc/trees/layer_tree_host_unittest_checkerimaging.cc[Added - diff]
• cc/trees/layer_tree_host_unittest_scroll.cc[diff]
• cc/trees/layer_tree_settings.h[diff]
• cc/trees/proxy_impl.cc[diff]
• cc/trees/single_thread_proxy.cc[diff]