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..947a8b6576c15fff434dcdda20085ba8325844b9 100644
--- a/cc/input/main_thread_scrolling_reason.h
+++ b/cc/input/main_thread_scrolling_reason.h
@@ -37,6 +37,7 @@ struct MainThreadScrollingReason {
     // These *AndLCDText reasons are due to subpixel text rendering which can
     // only be applied by blending glyphs with the background at a specific
     // screen position; transparency and transforms break this.
+    kNonCompositedReasonsFirst = 16,
     kHasOpacityAndLCDText = 1 << 16,
     kHasTransformAndLCDText = 1 << 17,
     kBackgroundNotOpaqueInRectAndLCDText = 1 << 18,
@@ -44,6 +45,14 @@ struct MainThreadScrollingReason {
     kHasClipRelatedProperty = 1 << 20,
     kHasBoxShadowFromNonRootLayer = 1 << 21,
 
+    // Scrolling a composited element which is on top of a non-composited
+    // 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.

[bokan, 2017/04/06 14:25:24]

Nit: the reason isn't added, it's reported.

+    kUnknownNonCompositedRegion = 1 << 22,
+    kNonCompositedReasonsLast = 22,
+
     // Transient scrolling reasons. These are computed for each scroll begin.
     kNonFastScrollableRegion = 1 << 5,
     kFailedHitTest = 1 << 7,
@@ -57,7 +66,7 @@ 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 +76,7 @@ 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 +90,20 @@ struct MainThreadScrollingReason {
     return (reasons & reasons_set_by_compositor) == reasons;
   }
 
+  // Usually a scroller won't be composited with the following reasons
+  // unless we force the promotion. e.g. Apply will-change:transform.
+  // Returns true if the MainThreadScrollingReason is one of the follow
+  // reasons that prevented a scroller from being composited.
+  static bool NonCompositedScrollReasons(uint32_t reasons) {
+    uint32_t non_composited_reasons =
+        kHasOpacityAndLCDText | kHasTransformAndLCDText |
+        kBackgroundNotOpaqueInRectAndLCDText | kHasBorderRadius |
+        kHasClipRelatedProperty | kHasBoxShadowFromNonRootLayer |
+        kUnknownNonCompositedRegion;
+    return reasons &&
+           (non_composited_reasons == (reasons | non_composited_reasons));
+  }
+
   static std::string mainThreadScrollingReasonsAsText(uint32_t reasons) {
     base::trace_event::TracedValue tracedValue;
     mainThreadScrollingReasonsAsTracedValue(reasons, &tracedValue);
@@ -131,6 +151,12 @@ struct MainThreadScrollingReason {
     if (reasons & MainThreadScrollingReason::kHasBoxShadowFromNonRootLayer)
       tracedValue->AppendString("Has box shadow from non-root layer");
 
+    // This reason is only used for reporting to UMA that we don't know why
+    // but we ended up on the main thread because of some non-composited
+    // scroller. It should never actually be stored/set on a ScrollableArea
+    // or GraphicsLayer therefore we don't need to trace it.
+    DCHECK(!(reasons & MainThreadScrollingReason::kUnknownNonCompositedRegion));
+
     // Transient scrolling reasons.
     if (reasons & MainThreadScrollingReason::kNonFastScrollableRegion)
       tracedValue->AppendString("Non fast scrollable region");
@@ -148,20 +174,6 @@ 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