Make SingleThreadProxy a SchedulerClient

All compositors that use SingleThreadProxy are left calling composite
synchronously and now pass a flag to indicate that this is their
intention.  This patch doesn't remove synchronous composite, but now
makes it mutually exclusive with scheduling.

Only cc unittests are exercising this code at this point, although it
also unifies a lot of the code for CompositeImmediately with the
scheduled path.

BUG=329552, 287250

Review URL: https://codereview.chromium.org/134623005

Cr-Commit-Position: refs/heads/master@{#291451}
git-svn-id: svn://svn.chromium.org/chrome/trunk/src@291451 0039d316-1c4b-4281-b951-d872f2087c98

• android_webview/browser/hardware_renderer.cc[diff]
• android_webview/browser/hardware_renderer.h[diff]
• cc/BUILD.gn[diff]
• cc/cc.gyp[diff]
• cc/scheduler/scheduler.cc[diff]
• cc/scheduler/scheduler.h[diff]
• cc/test/fake_layer_tree_host_client.h[diff]
• cc/test/layer_tree_test.cc[diff]
• cc/test/layer_tree_test.h[diff]
• cc/trees/layer_tree_host.cc[diff]
• cc/trees/layer_tree_host_perftest.cc[diff]
• cc/trees/layer_tree_host_single_thread_client.h[diff]
• cc/trees/layer_tree_host_unittest.cc[diff]
• cc/trees/layer_tree_host_unittest_context.cc[diff]
• cc/trees/layer_tree_host_unittest_no_message_loop.cc[diff]
• cc/trees/layer_tree_settings.cc[diff]
• cc/trees/layer_tree_settings.h[diff]
• cc/trees/scoped_abort_remaining_swap_promises.h[Added - diff]
• cc/trees/single_thread_proxy.cc[diff]
• cc/trees/single_thread_proxy.h[diff]
• cc/trees/thread_proxy.cc[diff]
• content/browser/renderer_host/compositor_impl_android.cc[diff]
• content/public/common/content_switches.cc[diff]
• content/public/common/content_switches.h[diff]
• content/renderer/gpu/render_widget_compositor.cc[diff]
• content/renderer/gpu/render_widget_compositor.h[diff]
• content/renderer/render_widget.cc[diff]
• content/renderer/render_widget.h[diff]
• content/shell/renderer/test_runner/web_test_proxy.cc[diff]
• content/test/web_layer_tree_view_impl_for_testing.h[diff]
• ui/compositor/compositor.cc[diff]