cc/blimp: Set up the framework for LayerTreeTests in remote mode.

This sets up the LayerTreeHostRemoteForTesting class which can be used
to mock out the external transport between the RemoteCompositorBridge
on the engine and LayerTreeHostInProcess on the client. The class
implements a simple binding layer that passes through state updates
between these 2.

BUG=653371
CQ_INCLUDE_TRYBOTS=master.tryserver.blink:linux_precise_blink_rel

Review-Url: https://codereview.chromium.org/2401703002
Cr-Commit-Position: refs/heads/master@{#424299}

• cc/BUILD.gn[diff]
• cc/blimp/layer_tree_host_remote.cc[diff]
• cc/blimp/layer_tree_host_remote.h[diff]
• cc/test/layer_tree_host_remote_for_testing.cc[Added - diff]
• cc/test/layer_tree_host_remote_for_testing.h[Added - diff]
• cc/test/layer_tree_test.cc[diff]
• cc/test/layer_tree_test.h[diff]
• cc/trees/layer_tree_host_unittest.cc[diff]
• cc/trees/layer_tree_host_unittest_proxy.cc[diff]