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

cc/input/main_thread_scrolling_reason.h

 // Copyright 2016 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef CC_INPUT_MAIN_THREAD_SCROLLING_REASON_H_
 #define CC_INPUT_MAIN_THREAD_SCROLLING_REASON_H_
 
 #include <string>
 
 #include "base/stl_util.h"
@@ skipping 49 matching lines @@
     kMainThreadScrollingReasonCount = 22,
   };
 
   // Returns true if the given MainThreadScrollingReason can be set by the main
   // thread.
   static bool MainThreadCanSetScrollReasons(uint32_t reasons) {
     uint32_t reasons_set_by_main_thread =
         kNotScrollingOnMain | kHasBackgroundAttachmentFixedObjects |
         kHasNonLayerViewportConstrainedObjects | kThreadedScrollingDisabled |
         kScrollbarScrolling | kPageOverlay | kHandlingScrollFromMainThread |
-        kCustomScrollbarScrolling | kHasOpacityAndLCDText |
-        kHasTransformAndLCDText | kBackgroundNotOpaqueInRectAndLCDText |
-        kHasBorderRadius | kHasClipRelatedProperty |
-        kHasBoxShadowFromNonRootLayer;
+        kCustomScrollbarScrolling;
     return (reasons & reasons_set_by_main_thread) == reasons;
   }
 
   // Returns true if the given MainThreadScrollingReason can be set by the
   // compositor.
   static bool CompositorCanSetScrollReasons(uint32_t reasons) {
     uint32_t reasons_set_by_compositor =
         kNonFastScrollableRegion | kFailedHitTest | kNoScrollingLayer |
         kNotScrollable | kContinuingMainThreadScroll | kNonInvertibleTransform |
         kPageBasedScrolling;
     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);
     std::string result_in_array_foramt = tracedValue.ToString();
     // Remove '{main_thread_scrolling_reasons:[', ']}', and any '"' chars.
     std::string result =
         result_in_array_foramt.substr(34, result_in_array_foramt.length() - 36);
     base::Erase(result, '\"');
     return result;
   }
@@ skipping 44 matching lines @@
     if (reasons & MainThreadScrollingReason::kNotScrollable)
       tracedValue->AppendString("Not scrollable");
     if (reasons & MainThreadScrollingReason::kContinuingMainThreadScroll)
       tracedValue->AppendString("Continuing main thread scroll");
     if (reasons & MainThreadScrollingReason::kNonInvertibleTransform)
       tracedValue->AppendString("Non-invertible transform");
     if (reasons & MainThreadScrollingReason::kPageBasedScrolling)
       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
 
 #endif  // CC_INPUT_MAIN_THREAD_SCROLLING_REASON_H_