Attempt to fix flaky ChromeOS test timeouts.

cc/scheduler/scheduler_state_machine.cc

Index: cc/scheduler/scheduler_state_machine.cc
diff --git a/cc/scheduler/scheduler_state_machine.cc b/cc/scheduler/scheduler_state_machine.cc
index 4f6d43f0d8bb7646ef8b77b16624817364a5a140..a6904ae2c6781341ced4067120f2089a43d740f1 100644
--- a/cc/scheduler/scheduler_state_machine.cc
+++ b/cc/scheduler/scheduler_state_machine.cc
@@ -320,7 +320,7 @@ bool SchedulerStateMachine::ShouldBeginOutputSurfaceCreation() const {
   if (begin_impl_frame_state_ != BEGIN_IMPL_FRAME_STATE_IDLE)
     return false;
 
-  // We want to clear the pipeline of any pending draws and activations
+  // We want to clear the pipline of any pending draws and activations
   // before starting output surface initialization. This allows us to avoid
   // weird corner cases where we abort draws or force activation while we
   // are initializing the output surface.
@@ -335,7 +335,7 @@ bool SchedulerStateMachine::ShouldBeginOutputSurfaceCreation() const {
 bool SchedulerStateMachine::ShouldDraw() const {
   // If we need to abort draws, we should do so ASAP since the draw could
   // be blocking other important actions (like output surface initialization),
-  // from occurring. If we are waiting for the first draw, then perform the
+  // from occuring. If we are waiting for the first draw, then perfom the
   // aborted draw to keep things moving. If we are not waiting for the first
   // draw however, we don't want to abort for no reason.
   if (PendingDrawsShouldBeAborted())
@@ -474,6 +474,14 @@ bool SchedulerStateMachine::ShouldSendBeginMainFrame() const {
   if (!HasInitializedOutputSurface())
     return false;
 
+  // SwapAck throttle the BeginMainFrames unless we just swapped.
+  // TODO(brianderson): Remove this restriction to improve throughput.
+  bool just_swapped_in_deadline =
+      begin_impl_frame_state_ == BEGIN_IMPL_FRAME_STATE_INSIDE_DEADLINE &&
+      HasSwappedThisFrame();
+  if (pending_swaps_ >= max_pending_swaps_ && !just_swapped_in_deadline)
+    return false;
+
   if (skip_begin_main_frame_to_reduce_latency_)
     return false;