Add transform and background not opaque as main thread scrolling reasons

cc/input/main_thread_scrolling_reason.h

Index: cc/input/main_thread_scrolling_reason.h
diff --git a/cc/input/main_thread_scrolling_reason.h b/cc/input/main_thread_scrolling_reason.h
index 191318b60a336982efaff0e489ffcd56245033ce..85a8ea117b7f1df92bf94ec792e3993599026bc6 100644
--- a/cc/input/main_thread_scrolling_reason.h
+++ b/cc/input/main_thread_scrolling_reason.h
@@ -32,6 +32,8 @@ struct MainThreadScrollingReason {
     kHandlingScrollFromMainThread = 1 << 13,
     kCustomScrollbarScrolling = 1 << 15,
     kHasOpacity = 1 << 16,
+    kHasTransform = 1 << 17,
+    kBackgroundNotOpaqueInRect = 1 << 18,
 
     // Transient scrolling reasons. These are computed for each scroll begin.
     kNonFastScrollableRegion = 1 << 5,
@@ -43,7 +45,7 @@ struct MainThreadScrollingReason {
     kPageBasedScrolling = 1 << 12,
 
     // The number of flags in this struct (excluding itself).
-    kMainThreadScrollingReasonCount = 18,
+    kMainThreadScrollingReasonCount = 20,

[ajuma, 2017/01/03 22:31:05]

Should this be 19? (since the flag with the largest value is 1 << 18)

[yigu, 2017/01/03 22:44:15]

It should be 19 + 1. I read about the reason somewhere. Will try to find it n
and keep you updated.

[yigu, 2017/01/03 23:09:14]

See https://codereview.chromium.org/2360113003 for the discussion. 

[ajuma, 2017/01/04 00:28:10]

Thanks for looking into this.

   };
 
   // Returns true if the given MainThreadScrollingReason can be set by the main
@@ -53,7 +55,8 @@ struct MainThreadScrollingReason {
         kNotScrollingOnMain | kHasBackgroundAttachmentFixedObjects |
         kHasNonLayerViewportConstrainedObjects | kThreadedScrollingDisabled |
         kScrollbarScrolling | kPageOverlay | kHandlingScrollFromMainThread |
-        kCustomScrollbarScrolling | kHasOpacity;
+        kCustomScrollbarScrolling | kHasOpacity | kHasTransform |
+        kBackgroundNotOpaqueInRect;
     return (reasons & reasons_set_by_main_thread) == reasons;
   }
 
@@ -100,6 +103,10 @@ struct MainThreadScrollingReason {
       tracedValue->AppendString("Custom scrollbar scrolling");
     if (reasons & MainThreadScrollingReason::kHasOpacity)
       tracedValue->AppendString("Has opacity");
+    if (reasons & MainThreadScrollingReason::kHasTransform)
+      tracedValue->AppendString("Has transform");
+    if (reasons & MainThreadScrollingReason::kBackgroundNotOpaqueInRect)
+      tracedValue->AppendString("Background is not opaque in rect");
 
     // Transient scrolling reasons.
     if (reasons & MainThreadScrollingReason::kNonFastScrollableRegion)