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 19 matching lines @@
     // blink::ScrollAnimator is in the middle of running a scroll offset
     // animation. Note that a scroll handled by the main thread can result in an
     // animation running on the main thread or on the compositor thread.
     kHandlingScrollFromMainThread = 1 << 13,
     kCustomScrollbarScrolling = 1 << 15,
 
     // Style-related scrolling on main reasons.
     // 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,
     kHasBorderRadius = 1 << 19,
     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,
     kNoScrollingLayer = 1 << 8,
     kNotScrollable = 1 << 9,
     kContinuingMainThreadScroll = 1 << 10,
     kNonInvertibleTransform = 1 << 11,
     kPageBasedScrolling = 1 << 12,
 
     // The maximum number of flags in this struct (excluding itself).
     // 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
   // 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;
   }
 
+  // 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);
     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 27 matching lines @@
       tracedValue->AppendString(
           "Background is not opaque in rect and LCD text");
     }
     if (reasons & MainThreadScrollingReason::kHasBorderRadius)
       tracedValue->AppendString("Has border radius");
     if (reasons & MainThreadScrollingReason::kHasClipRelatedProperty)
       tracedValue->AppendString("Has clip related property");
     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");
     if (reasons & MainThreadScrollingReason::kFailedHitTest)
       tracedValue->AppendString("Failed hit test");
     if (reasons & MainThreadScrollingReason::kNoScrollingLayer)
       tracedValue->AppendString("No scrolling layer");
     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_