Fold BeginFrameSource::SetClientReady into AddObserver

cc/test/fake_external_begin_frame_source.cc

Index: cc/test/fake_external_begin_frame_source.cc
diff --git a/cc/test/fake_external_begin_frame_source.cc b/cc/test/fake_external_begin_frame_source.cc
index 1875bdbd2da91db846551c5a07793e76f6bfac3d..fee842b25604166fa94a9aceaa35c46cf62dda67 100644
--- a/cc/test/fake_external_begin_frame_source.cc
+++ b/cc/test/fake_external_begin_frame_source.cc
@@ -14,7 +14,6 @@ namespace cc {
 
 FakeExternalBeginFrameSource::FakeExternalBeginFrameSource(double refresh_rate)
     : milliseconds_per_frame_(1000.0 / refresh_rate),
-      is_ready_(false),
       weak_ptr_factory_(this) {
   DetachFromThread();
 }
@@ -23,11 +22,6 @@ FakeExternalBeginFrameSource::~FakeExternalBeginFrameSource() {
   DCHECK(CalledOnValidThread());
 }
 
-void FakeExternalBeginFrameSource::SetClientReady() {
-  DCHECK(CalledOnValidThread());
-  is_ready_ = true;
-}
-
 void FakeExternalBeginFrameSource::OnNeedsBeginFramesChanged(
     bool needs_begin_frames) {
   DCHECK(CalledOnValidThread());