cc: Ensure Scheduler's deadline_mode_ is initialized to a valid value.

Otherwise we can end up hitting DCHECKs in e.g. 
BeginImplFrameDeadlineModeToString().

Change-Id: Ibaac3356ba94664ce9c08d378ab78631e8aaa890
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2196582Reviewed-by: Sunny Sachanandani <sunnyps@chromium.org>
Commit-Queue: Eric Seckler <eseckler@chromium.org>
Cr-Commit-Position: refs/heads/master@{#771125}

cc/scheduler/scheduler.h

@@ -281,7 +281,8 @@ class CC_EXPORT Scheduler : public viz::BeginFrameObserverBase {
   // What the latest deadline was, and when it was scheduled.
   base::TimeTicks deadline_;
   base::TimeTicks deadline_scheduled_at_;
-  SchedulerStateMachine::BeginImplFrameDeadlineMode deadline_mode_;
+  SchedulerStateMachine::BeginImplFrameDeadlineMode deadline_mode_ =
+      SchedulerStateMachine::BeginImplFrameDeadlineMode::NONE;
 
   BeginFrameTracker begin_impl_frame_tracker_;
   viz::BeginFrameAck last_begin_frame_ack_;