Refactoring the way begin frame sources inside scheduler work.

cc/BUILD.gn

Index: cc/BUILD.gn
diff --git a/cc/BUILD.gn b/cc/BUILD.gn
index e79753175e6e428375b43a88205cd460a0e7869a..0515a6ff954919b0fae35f5f4cab70175af2c6af 100644
--- a/cc/BUILD.gn
+++ b/cc/BUILD.gn
@@ -279,6 +279,7 @@ component("cc") {
     "output/software_renderer.h",
     "output/viewport_selection_bound.cc",
     "output/viewport_selection_bound.h",
+    "output/vsync_parameter_observer.h",
     "quads/checkerboard_draw_quad.cc",
     "quads/checkerboard_draw_quad.h",
     "quads/content_draw_quad_base.cc",
@@ -430,6 +431,8 @@ component("cc") {
     "resources/video_resource_updater.h",
     "resources/zero_copy_raster_worker_pool.cc",
     "resources/zero_copy_raster_worker_pool.h",
+    "scheduler/begin_frame_source.cc",
+    "scheduler/begin_frame_source.h",
     "scheduler/delay_based_time_source.cc",
     "scheduler/delay_based_time_source.h",
     "scheduler/draw_result.h",
@@ -733,6 +736,7 @@ test("cc_unittests") {
     "resources/tile_manager_unittest.cc",
     "resources/tile_priority_unittest.cc",
     "resources/video_resource_updater_unittest.cc",
+    "scheduler/begin_frame_source_unittest.cc",
     "scheduler/delay_based_time_source_unittest.cc",
     "scheduler/scheduler_state_machine_unittest.cc",
     "scheduler/scheduler_unittest.cc",