Moves ownership of the cc::Display's BeginFrameSource out of Display.

cc/surfaces/display_scheduler.cc

Index: cc/surfaces/display_scheduler.cc
diff --git a/cc/surfaces/display_scheduler.cc b/cc/surfaces/display_scheduler.cc
index 229bc6306ccb1ecf4af4056e074b35a56ac9f429..46f26ff331f2306d8f504b58f907fbc060d0affd 100644
--- a/cc/surfaces/display_scheduler.cc
+++ b/cc/surfaces/display_scheduler.cc
@@ -13,10 +13,10 @@
 
 namespace cc {
 
-DisplayScheduler::DisplayScheduler(BeginFrameSource* begin_frame_source,
-                                   base::SingleThreadTaskRunner* task_runner,
+DisplayScheduler::DisplayScheduler(base::SingleThreadTaskRunner* task_runner,
                                    int max_pending_swaps)
-    : begin_frame_source_(begin_frame_source),
+    : client_(nullptr),

[brianderson, 2016/12/09 19:01:57]

Thanks for fixing this!

+      begin_frame_source_(nullptr),
       task_runner_(task_runner),
       inside_surface_damaged_(false),
       visible_(false),
@@ -44,6 +44,11 @@ void DisplayScheduler::SetClient(DisplaySchedulerClient* client) {
   client_ = client;
 }
 
+void DisplayScheduler::SetBeginFrameSource(

[brianderson, 2016/12/09 19:01:57]

For my understanding, was were the reasons to use SetBeginFrameSource rather
than passing it in to the constructor?

I see that it is:
Clearer from Display that its DisplayScheduler will have the same
BeginFrameSource.
Sets the DisplayScheduler's BeginFrameSource from fewer places.

Any others?

[Eric Seckler, 2016/12/12 09:27:18]

What you said :) Also, it's useful for later wrapping the Display's BFS  (with
something that can keep track of outstanding BeginFrames / observer responses,
see DisplayBeginFrameSource in https://codereview.chromium.org/2527283003/) and
providing the wrapped source to the scheduler.

+    BeginFrameSource* begin_frame_source) {
+  begin_frame_source_ = begin_frame_source;
+}
+
 void DisplayScheduler::SetVisible(bool visible) {
   if (visible_ == visible)
     return;