Record non-stacking-context 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 a8c586eee95ab06c830406147aae2ebab941929d..b3fdf1e86bf59d7c7d6de3c64694992f535b1b4f 100644
--- a/cc/input/main_thread_scrolling_reason.h
+++ b/cc/input/main_thread_scrolling_reason.h
@@ -44,7 +44,9 @@ struct MainThreadScrollingReason {
     kHasBorderRadius = 1 << 19,
     kHasClipRelatedProperty = 1 << 20,
     kHasBoxShadowFromNonRootLayer = 1 << 21,
-    kNonCompositedReasonsLast = 21,
+    kIsNotStackingContextAndLCDText = 1 << 22,
+    kHasOverflowClipAndLCDText = 1 << 23,

[bokan, 2017/04/25 18:04:15]

I think this name should be more specific - having an overflow clip isn't enough
to have this reason set.

[yigu, 2017/04/26 15:38:36]

Updated. It also requires that the layout object's background is not opaque,
e.g.  it has alpha channel.

+    kNonCompositedReasonsLast = 23,
 
     // Transient scrolling reasons. These are computed for each scroll begin.
     kNonFastScrollableRegion = 1 << 5,
@@ -59,13 +61,14 @@ struct MainThreadScrollingReason {
     // New flags should increment this number but it should never be decremented
     // because the values are used in UMA histograms. It should also be noted
     // that it excludes the kNotScrollingOnMain value.
-    kMainThreadScrollingReasonCount = 22,
+    kMainThreadScrollingReasonCount = 24,
   };
 
   static const uint32_t kNonCompositedReasons =
       kHasOpacityAndLCDText | kHasTransformAndLCDText |
       kBackgroundNotOpaqueInRectAndLCDText | kHasBorderRadius |
-      kHasClipRelatedProperty | kHasBoxShadowFromNonRootLayer;
+      kHasClipRelatedProperty | kHasBoxShadowFromNonRootLayer |
+      kIsNotStackingContextAndLCDText | kHasOverflowClipAndLCDText;
 
   // Returns true if the given MainThreadScrollingReason can be set by the main
   // thread.
@@ -140,6 +143,10 @@ struct MainThreadScrollingReason {
       tracedValue->AppendString("Has clip related property");
     if (reasons & MainThreadScrollingReason::kHasBoxShadowFromNonRootLayer)
       tracedValue->AppendString("Has box shadow from non-root layer");
+    if (reasons & MainThreadScrollingReason::kIsNotStackingContextAndLCDText)
+      tracedValue->AppendString("Is not stacking context and LCD text");
+    if (reasons & MainThreadScrollingReason::kHasOverflowClipAndLCDText)
+      tracedValue->AppendString("Has overflow clip and LCD text");
 
     // Transient scrolling reasons.
     if (reasons & MainThreadScrollingReason::kNonFastScrollableRegion)