Refactoring the way begin frame sources inside scheduler work.

This change;
 * Makes non-vsync aligned rendering just another begin frame source.
 * Makes it easier to add vsync/begin frame stabilisation / filtering in the future.

This CL no longer moves background ticking into the scheduler rather than the LayerTreeHostImpl, that will occur in a later CL.

BUG=345459

Review URL: https://codereview.chromium.org/267783004

Cr-Commit-Position: refs/heads/master@{#297389}

• cc/BUILD.gn[diff]
• cc/cc.gyp[diff]
• cc/cc_tests.gyp[diff]
• cc/output/begin_frame_args.cc[diff]
• cc/output/begin_frame_args.h[diff]
• cc/output/vsync_parameter_observer.h[Added - diff]
• cc/scheduler/begin_frame_source.cc[Added - diff]
• cc/scheduler/begin_frame_source.h[Added - diff]
• cc/scheduler/begin_frame_source_unittest.cc[Added - diff]
• cc/scheduler/delay_based_time_source.h[diff]
• cc/scheduler/scheduler.cc[diff]
• cc/scheduler/scheduler.h[diff]
• cc/scheduler/scheduler_unittest.cc[diff]
• cc/test/begin_frame_args_test.cc[diff]
• cc/test/begin_frame_args_test.h[diff]
• cc/test/fake_layer_tree_host_impl_client.h[diff]
• cc/test/ordered_simple_task_runner.cc[diff]
• cc/test/ordered_simple_task_runner.h[diff]
• cc/test/scheduler_test_common.cc[diff]
• cc/test/scheduler_test_common.h[diff]
• cc/trees/layer_tree_host_impl.cc[diff]
• cc/trees/layer_tree_host_impl.h[diff]
• cc/trees/layer_tree_host_impl_unittest.cc[diff]
• cc/trees/single_thread_proxy.cc[diff]
• cc/trees/single_thread_proxy.h[diff]
• cc/trees/thread_proxy.cc[diff]
• cc/trees/thread_proxy.h[diff]