Add begin frame paused signal

This avoids a deadlock situation in android webview where
begin frames have stopped, but the blink main thread is blocked
indefinitely waiting for activation. See bug for details.

Plumb a signal that the BeginFrameSource is paused directly
through BeginFrameSourceObserver, and force activate any
pending activations when paused. This is similar to when
compositor becomes invisible, but BFS allows webview send
the signal without a hop to blink main thread.

BUG=539373
CQ_INCLUDE_TRYBOTS=tryserver.blink:linux_blink_rel

Review URL: https://codereview.chromium.org/1536353003

Cr-Commit-Position: refs/heads/master@{#369238}

• cc/scheduler/begin_frame_source.cc[diff]
• cc/scheduler/begin_frame_source.h[diff]
• cc/scheduler/begin_frame_source_unittest.cc[diff]
• cc/scheduler/scheduler.cc[diff]
• cc/scheduler/scheduler.h[diff]
• cc/scheduler/scheduler_state_machine.cc[diff]
• cc/scheduler/scheduler_state_machine.h[diff]
• cc/scheduler/scheduler_state_machine_unittest.cc[diff]
• cc/scheduler/scheduler_unittest.cc[diff]
• cc/surfaces/display_scheduler.cc[diff]
• cc/surfaces/display_scheduler.h[diff]
• cc/test/begin_frame_source_test.h[diff]
• cc/test/scheduler_test_common.h[diff]
• content/browser/android/in_process/synchronous_compositor_impl.cc[diff]
• content/browser/android/synchronous_compositor_host.cc[diff]
• content/common/android/sync_compositor_messages.cc[diff]
• content/common/android/sync_compositor_messages.h[diff]
• content/renderer/android/synchronous_compositor_external_begin_frame_source.cc[diff]
• content/renderer/android/synchronous_compositor_external_begin_frame_source.h[diff]
• content/renderer/android/synchronous_compositor_proxy.cc[diff]