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 299a0cd5b3ebb64d630d91d24cbee179ecf082c0..b3751111043bc5ee36f9541e759a5960d71a8a59 100644
--- a/cc/BUILD.gn
+++ b/cc/BUILD.gn
@@ -431,6 +431,8 @@ component("cc") {
     "scheduler/delay_based_time_source.cc",
     "scheduler/delay_based_time_source.h",
     "scheduler/draw_result.h",
+    "scheduler/frame_source.cc",
+    "scheduler/frame_source.h",
     "scheduler/scheduler.cc",
     "scheduler/scheduler.h",
     "scheduler/scheduler_settings.cc",
@@ -733,6 +735,7 @@ test("cc_unittests") {
     "resources/tile_priority_unittest.cc",
     "resources/video_resource_updater_unittest.cc",
     "scheduler/delay_based_time_source_unittest.cc",
+    "scheduler/frame_source_unittest.cc",
     "scheduler/scheduler_state_machine_unittest.cc",
     "scheduler/scheduler_unittest.cc",
     "test/layer_tree_json_parser_unittest.cc",