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..6088656e8f5b81cd67c3df189f0eaf822c0c515c 100644
--- a/cc/input/main_thread_scrolling_reason.h
+++ b/cc/input/main_thread_scrolling_reason.h
@@ -67,10 +67,7 @@ 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 +81,16 @@ struct MainThreadScrollingReason {
     return (reasons & reasons_set_by_compositor) == reasons;
   }
 
+  // Returns true if the given MainThreadScrollingReason is non zero and
+  // contains any of the following non composited scroll reasons.

[bokan, 2017/03/28 15:30:15]

The comment doesn't add anything - it says the exact same thing as the code.
Describe "why"/"what", not "how". In this case, I'd say that these are the
"reasons that prevent a scroller from being composited" or something along those
lines.

+  static bool NonCompositedScrollReasons(uint32_t reasons) {
+    uint32_t reasons_set_by_main_thread_only =

[bokan, 2017/03/28 15:30:15]

reasons_set_by_main_thread_only -> non_composited_reasons

+        kHasOpacityAndLCDText | kHasTransformAndLCDText |
+        kBackgroundNotOpaqueInRectAndLCDText | kHasBorderRadius |
+        kHasClipRelatedProperty | kHasBoxShadowFromNonRootLayer;
+    return reasons && (reasons & reasons_set_by_main_thread_only);
+  }
+
   static std::string mainThreadScrollingReasonsAsText(uint32_t reasons) {
     base::trace_event::TracedValue tracedValue;
     mainThreadScrollingReasonsAsTracedValue(reasons, &tracedValue);
@@ -148,20 +155,6 @@ 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