cc: Split ThreadProxy into ProxyMain and ProxyImpl

This is the final patch that splits ThreadProxy into ProxyMain and
ProxyImpl routing all inter-proxy communication using ChannelMain
and ChannelImpl.

ThreadProxy currently implements the logic for glueing together the
Scheduler, LayerTreeHostImpl and the LayerTreeHost and separating these
components across the main and impl thread boundary. This patch isolates
the logic for this glue code in ThreadProxy exclusive to each thread to
ProxyMain and ProxyImpl. This will allow us to abstract the medium the
2 sides use to communicate with each other so these components can be
run across a thread/process/network boundary.

ThreadedChannel implements the in-process threaded case.

BUG=527200
CQ_INCLUDE_TRYBOTS=tryserver.blink:linux_blink_rel

Review URL: https://codereview.chromium.org/1417053005

Cr-Commit-Position: refs/heads/master@{#364034}

• cc/BUILD.gn[diff]
• cc/cc.gyp[diff]
• cc/cc_tests.gyp[diff]
• cc/debug/benchmark_instrumentation.h[diff]
• cc/scheduler/scheduler_state_machine_unittest.cc[diff]
• cc/test/layer_tree_test.cc[diff]
• cc/test/layer_tree_test.h[diff]
• cc/test/proxy_impl_for_test.cc[Added - diff]
• cc/test/proxy_impl_for_test.h[Added - diff]
• cc/test/proxy_main_for_test.cc[Added - diff]
• cc/test/proxy_main_for_test.h[Added - diff]
• cc/test/test_hooks.cc[Added - diff]
• cc/test/test_hooks.h[Added - diff]
• cc/test/threaded_channel_for_test.cc[Added - diff]
• cc/test/threaded_channel_for_test.h[Added - diff]
• cc/trees/channel_main.h[diff]
• cc/trees/layer_tree_host.cc[diff]
• cc/trees/layer_tree_host_unittest.cc[diff]
• cc/trees/layer_tree_host_unittest_proxy.cc[diff]
• cc/trees/proxy_impl.cc[Added - diff]
• cc/trees/proxy_impl.h[diff]
• cc/trees/proxy_main.cc[Added - diff]
• cc/trees/proxy_main.h[diff]
• cc/trees/thread_proxy.cc[Deleted - diff]
• cc/trees/thread_proxy.h[Deleted - diff]
• cc/trees/threaded_channel.cc[diff]
• cc/trees/threaded_channel.h[diff]
• cc/trees/threaded_channel_unittest.cc[diff]