Fix smooth scroll jump when switching scroll handling between MT and CC

cc/layers/layer.cc

Index: cc/layers/layer.cc
diff --git a/cc/layers/layer.cc b/cc/layers/layer.cc
index 24ae93291ebf13f2f225c3c6231e8f084b03b736..b1ade9b6ed8869a2ed51a8cf359c722fb93638eb 100644
--- a/cc/layers/layer.cc
+++ b/cc/layers/layer.cc
@@ -960,12 +960,12 @@ void Layer::AddMainThreadScrollingReasons(
   SetNeedsCommit();
 }
 
-void Layer::ClearMainThreadScrollingReasons() {
+void Layer::ClearMainThreadScrollingReasons(
+    uint32_t main_thread_scrolling_reasons_to_clear) {
   DCHECK(IsPropertyChangeAllowed());
-  if (!main_thread_scrolling_reasons_)
-    return;
+  DCHECK(main_thread_scrolling_reasons_to_clear);

[ajuma, 2016/02/04 20:08:40]

Is it possible that we reach here and main_thread_scrolling_reasons_ and
main_thread_scrolling_reasons_to_clear have an empty intersection? (If so, we
should still check for that and early out to avoid an unnecessary commit.)

[ymalik, 2016/02/04 21:53:20]

Yeah totally. Perhaps we should do the same for AddMainThreadScrollingReasons
above?

[ajuma, 2016/02/04 22:45:29]

Yes, please do.

   main_thread_scrolling_reasons_ =
-      MainThreadScrollingReason::kNotScrollingOnMain;
+      ~main_thread_scrolling_reasons_to_clear & main_thread_scrolling_reasons_;
   SetNeedsCommit();
 }