cc: Make BackToBackBeginFrameSource a SyntheticBeginFrameSource.

cc/trees/proxy_impl.cc

Index: cc/trees/proxy_impl.cc
diff --git a/cc/trees/proxy_impl.cc b/cc/trees/proxy_impl.cc
index f0f1c71ec00506dfe48dc76b44cbecba1b0ea6a6..1bd55fcf6ddcf71e3935bd5cef84488f526a117b 100644
--- a/cc/trees/proxy_impl.cc
+++ b/cc/trees/proxy_impl.cc
@@ -19,6 +19,7 @@
 #include "cc/output/context_provider.h"
 #include "cc/output/output_surface.h"
 #include "cc/scheduler/compositor_timing_history.h"
+#include "cc/scheduler/delay_based_time_source.h"
 #include "cc/trees/layer_tree_host.h"
 #include "cc/trees/layer_tree_impl.h"
 #include "cc/trees/task_runner_provider.h"
@@ -90,9 +91,9 @@ ProxyImpl::ProxyImpl(
     frame_source = unthrottled_begin_frame_source_.get();
   }
   if (!frame_source) {
-    synthetic_begin_frame_source_.reset(new SyntheticBeginFrameSource(
-        task_runner_provider_->ImplThreadTaskRunner(),
-        BeginFrameArgs::DefaultInterval()));
+    synthetic_begin_frame_source_.reset(
+        new SyntheticBeginFrameSource(base::MakeUnique<DelayBasedTimeSource>(
+            task_runner_provider_->ImplThreadTaskRunner())));
     frame_source = synthetic_begin_frame_source_.get();
   }
   scheduler_ =
@@ -305,14 +306,8 @@ void ProxyImpl::DidLoseOutputSurfaceOnImplThread() {
 void ProxyImpl::CommitVSyncParameters(base::TimeTicks timebase,
                                       base::TimeDelta interval) {
   DCHECK(IsImplThread());
-  if (!synthetic_begin_frame_source_)
-    return;
-
-  if (interval.is_zero()) {
-    // TODO(brianderson): We should not be receiving 0 intervals.
-    interval = BeginFrameArgs::DefaultInterval();
-  }
-  synthetic_begin_frame_source_->OnUpdateVSyncParameters(timebase, interval);
+  if (synthetic_begin_frame_source_)
+    synthetic_begin_frame_source_->OnUpdateVSyncParameters(timebase, interval);
 }
 
 void ProxyImpl::SetBeginFrameSource(BeginFrameSource* source) {