Move logic of recording main thread scrolling reasons from cc to blink::ScrollManager

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 9746d9425740d96ff63e52707b498f08b4878d80..2de0041991d1f1a8b05b849a49cbda46de2974e4 100644
--- a/cc/input/main_thread_scrolling_reason.h
+++ b/cc/input/main_thread_scrolling_reason.h
@@ -43,6 +43,12 @@ struct MainThreadScrollingReason {
     kHasBorderRadius = 1 << 19,
     kHasClipRelatedProperty = 1 << 20,
     kHasBoxShadowFromNonRootLayer = 1 << 21,
+    // Scrolling a composited element which is on top of a non-composited

[bokan, 2017/03/31 16:55:52]

Nit: newline above

+    // one will end up with main thread scrolling because we are scrolling a non
+    // fast scrollable region. If all scrollers from the composited element's
+    // containing block chain are composited, it must be because of an element
+    // not in the chain in which case the following reason is added.
+    kUnknownNonCompositedRegion = 1 << 22,
 
     // Transient scrolling reasons. These are computed for each scroll begin.
     kNonFastScrollableRegion = 1 << 5,
@@ -57,7 +63,7 @@ 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 = 23,
   };
 
   // Returns true if the given MainThreadScrollingReason can be set by the main
@@ -67,10 +73,7 @@ struct MainThreadScrollingReason {
         kNotScrollingOnMain | kHasBackgroundAttachmentFixedObjects |
         kHasNonLayerViewportConstrainedObjects | kThreadedScrollingDisabled |
         kScrollbarScrolling | kPageOverlay | kHandlingScrollFromMainThread |
-        kCustomScrollbarScrolling | kHasOpacityAndLCDText |
-        kHasTransformAndLCDText | kBackgroundNotOpaqueInRectAndLCDText |
-        kHasBorderRadius | kHasClipRelatedProperty |
-        kHasBoxShadowFromNonRootLayer;
+        kCustomScrollbarScrolling;
     return (reasons & reasons_set_by_main_thread) == reasons;
   }
 
@@ -84,6 +87,17 @@ struct MainThreadScrollingReason {
     return (reasons & reasons_set_by_compositor) == reasons;
   }
 
+  // Usually a scroller won't be composited with the following reasons

[bokan, 2017/03/31 16:55:52]

If we force promotion though, we remove these reasons right?

[yigu, 2017/03/31 19:18:55]

Actually we don't remove these reasons for PLSA. If the PLSA is composited, we
won't record these reasons.

[bokan, 2017/04/04 18:14:26]

We won't *record* the reasons, but will they still be *set* on the PLSA itself?

I would also reword the comment to make the meaning clearer. e.g. "Returns true
if the MainThreadScrollingReason is one that prevented a scroller from being
composited". 

[yigu, 2017/04/05 20:18:02]

When composited, the reasons won't be set on PLSA.

+  // unless we force the promotion. e.g. Apply will-change:transform.
+  static bool NonCompositedScrollReasons(uint32_t reasons) {
+    uint32_t non_composited_reasons =
+        kHasOpacityAndLCDText | kHasTransformAndLCDText |
+        kBackgroundNotOpaqueInRectAndLCDText | kHasBorderRadius |
+        kHasClipRelatedProperty | kHasBoxShadowFromNonRootLayer |
+        kUnknownNonCompositedRegion;
+    return reasons && (reasons & non_composited_reasons);
+  }
+
   static std::string mainThreadScrollingReasonsAsText(uint32_t reasons) {
     base::trace_event::TracedValue tracedValue;
     mainThreadScrollingReasonsAsTracedValue(reasons, &tracedValue);
@@ -130,6 +144,8 @@ struct MainThreadScrollingReason {
       tracedValue->AppendString("Has clip related property");
     if (reasons & MainThreadScrollingReason::kHasBoxShadowFromNonRootLayer)
       tracedValue->AppendString("Has box shadow from non-root layer");
+    if (reasons & MainThreadScrollingReason::kUnknownNonCompositedRegion)

[bokan, 2017/03/31 16:55:52]

I think this should never actually be stored/set on a ScrollableArea or
GraphicsLayer, right? We basically only use it for reporting to UMA that we
don't know why but we ended up on the main thread because of *some*
non-composited scroller. In that case, perhaps we should just DCHECK here that
it's not this reason as I think this function is only used to print out reasons
attached to layers/areas.

+      tracedValue->AppendString("Unknown non composited region");
 
     // Transient scrolling reasons.
     if (reasons & MainThreadScrollingReason::kNonFastScrollableRegion)
@@ -148,20 +164,6 @@ struct MainThreadScrollingReason {
       tracedValue->AppendString("Page-based scrolling");
     tracedValue->EndArray();
   }
-
-  // For a given reason, return its index in enum
-  static int getReasonIndex(uint32_t reason) {
-    // Multiple reasons provided
-    if (reason & (reason - 1))
-      return -1;
-
-    int index = -1;
-    while (reason > 0) {
-      reason = reason >> 1;
-      ++index;
-    }
-    return index;
-  }
 };
 
 }  // namespace cc