Avoid DCHECK in ExternalBeginFrameSource::GetMissedBeginFrameArgs caused by out of order frame time

cc/scheduler/begin_frame_source_unittest.cc

Index: cc/scheduler/begin_frame_source_unittest.cc
diff --git a/cc/scheduler/begin_frame_source_unittest.cc b/cc/scheduler/begin_frame_source_unittest.cc
index a4cf4b59268e9b5aa201c6ea9a58ef0adee4ed11..d1800460f842cd88409cdd36fd99867e65902dc3 100644
--- a/cc/scheduler/begin_frame_source_unittest.cc
+++ b/cc/scheduler/begin_frame_source_unittest.cc
@@ -573,5 +573,31 @@ TEST_F(ExternalBeginFrameSourceTest, OnBeginFrameChecksBeginFrameContinuity) {
   source2.OnBeginFrame(args);
 }
 
+// https://crbug.com/730218: Avoid DCHECK crash in
+// ExternalBeginFrameSource::GetMissedBeginFrameArgs.
+TEST_F(ExternalBeginFrameSourceTest, GetMissedBeginFrameArgs) {
+  BeginFrameArgs args = CreateBeginFrameArgsForTesting(BEGINFRAME_FROM_HERE, 0,
+                                                       2, 10000, 10100, 100);
+  source_->OnBeginFrame(args);
+
+  EXPECT_BEGIN_FRAME_SOURCE_PAUSED(*obs_, false);
+  EXPECT_BEGIN_FRAME_USED_MISSED(*obs_, 0, 2, 10000, 10100, 100);
+  source_->AddObserver(obs_.get());
+  source_->RemoveObserver(obs_.get());
+
+  // Out of order frame_time. This might not be valid but still shouldn't
+  // cause a DCHECK in ExternalBeginFrameSource code.
+  args = CreateBeginFrameArgsForTesting(BEGINFRAME_FROM_HERE, 0, 2, 9999, 10100,
+                                        101);
+  source_->OnBeginFrame(args);
+
+  EXPECT_CALL((*client_), OnNeedsBeginFrames(true)).Times(1);
+  EXPECT_BEGIN_FRAME_SOURCE_PAUSED(*obs_, false);
+  EXPECT_CALL(*obs_, OnBeginFrame(CreateBeginFrameArgsForTesting(

[sunnyps, 2017/06/07 23:22:24]

nit: EXPECT_CALL(*obs_, _) instead? We don't want any begin frame no matter how
the args are calculated.

[stanisc, 2017/06/07 23:36:33]

Done. I tried that initially but couldn't make it work because I was
missing a using statement at the top.

+                         BEGINFRAME_FROM_HERE, 0, 1, 9999, 10100, 101)))
+      .Times(0);
+  source_->AddObserver(obs_.get());
+}
+
 }  // namespace
 }  // namespace cc