Add more specific metrics for main thread scrolling reasons

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/trace_event/trace_event_argument.h"
@@ skipping 13 matching lines @@
     kScrollbarScrolling = 1 << 3,
     kPageOverlay = 1 << 4,
 
     // This bit is set when any of the other main thread scrolling reasons cause
     // an input event to be handled on the main thread, and the main thread
     // 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,
+    kHasOpacity = 1 << 16,

[bokan, 2016/12/13 14:38:56]

kHasOpacityAndScrollDependent? A regular scroller won't be main threaded because
it's transparent, right?

[yigu, 2016/12/13 20:54:10]

It should scroll on main thread due to blending I guess.

[bokan, 2016/12/14 13:28:58]

Ah, of course, if it's transparent and low-DPI, we won't promote the scroller
because we'd lose LCD text. This means though you need to add a check when
setting the kHasOpacity that the scroller actually isn't composited. If you try
the current patch on a HiDPI monitor it'll scroll on impl but you'll set this
flag. You can do that by checking
PaintLayerCompositor::preferCompositingToLCDText, if that's false, a transparent
layer will lose composited scrolling.

[bokan, 2016/12/15 20:47:45]

Hey Yi, I think this still hasn't been addressed. I think a scroller with
opacity will scroll on impl thread if you're on a high-dpi monitor or pass
--prefer-compositing-to-lcd-text. Could you confirm that's true, and if so,
you'll need to adjust the condition under which we set this
MainThreadScrollingReason

 
     // 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 number of flags in this struct (excluding itself).
-    kMainThreadScrollingReasonCount = 17,
+    kMainThreadScrollingReasonCount = 18,
   };
 
   // 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;
+        kCustomScrollbarScrolling | kHasOpacity;
     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;
@@ skipping 24 matching lines @@
     if (reasons & MainThreadScrollingReason::kThreadedScrollingDisabled)
       tracedValue->AppendString("Threaded scrolling is disabled");
     if (reasons & MainThreadScrollingReason::kScrollbarScrolling)
       tracedValue->AppendString("Scrollbar scrolling");
     if (reasons & MainThreadScrollingReason::kPageOverlay)
       tracedValue->AppendString("Page overlay");
     if (reasons & MainThreadScrollingReason::kHandlingScrollFromMainThread)
       tracedValue->AppendString("Handling scroll from main thread");
     if (reasons & MainThreadScrollingReason::kCustomScrollbarScrolling)
       tracedValue->AppendString("Custom scrollbar scrolling");
+    if (reasons & MainThreadScrollingReason::kHasOpacity)
+      tracedValue->AppendString("Has opacity");
 
     // 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();
   }
 };
 
 }  // namespace cc
 
 #endif  // CC_INPUT_MAIN_THREAD_SCROLLING_REASON_H_