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..639120295036dc10d53775bae3f919051070a0e0 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,
+    kHasOverflowClipAndLayoutObjectBackgroundNotOpaqueAndLCDText = 1 << 23,

[bokan, 2017/04/26 15:52:15]

I'd just call it kHasOverflowClipNotOpaqueAndLCDText, LayoutObject is implied.

[yigu, 2017/04/27 15:28:27]

We have another BackgroundNotOpaque used for PaintLayer (see reason 18 above).
That's why I had this lame name.

[bokan, 2017/04/27 15:40:31]

But the difference between the two is that this reason also adds
HasOverflowClip, right? For my understanding: this new reason is  basically a
specialisation of BackgroundNotOpaque, right? So if this reason is set,
BackgroundNotOpaque will also be set. Is that right?

[flackr, 2017/04/27 18:42:35]

I thought all of these were reasons not to promote overflow scrollers? The
function seems to be called from
PaintLayerScrollableArea::ComputeNeedsCompositedScrolling which has an early out
if it's not a scroller. So BackgroundNotOpaque also has overflow, in which case
I'm not sure I understand the difference here. When is BackgroundNotOpaque true
but this false, or vice versa?

Looking at where you added it, I think the condition you mean to add is just
Clip, not specifically overflow clip, but because we're only recording this
metric for overflow scrollers which we are considering reporting it will always
have a clip related property so all we can say is that we don't know if the
background will be opaque (i.e. BackgroundNotOpaque).

[yigu, 2017/04/28 14:46:05]

This reason has been removed from this patch.

+    kNonCompositedReasonsLast = 23,
 
     // Transient scrolling reasons. These are computed for each scroll begin.
     kNonFastScrollableRegion = 1 << 5,
@@ -59,13 +61,15 @@ 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 |
+      kHasOverflowClipAndLayoutObjectBackgroundNotOpaqueAndLCDText;
 
   // Returns true if the given MainThreadScrollingReason can be set by the main
   // thread.
@@ -140,6 +144,14 @@ 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::
+            kHasOverflowClipAndLayoutObjectBackgroundNotOpaqueAndLCDText)
+      tracedValue->AppendString(
+          "Has overflow clip and layout object background not opaque and LCD "
+          "text");
 
     // Transient scrolling reasons.
     if (reasons & MainThreadScrollingReason::kNonFastScrollableRegion)