Add more specific metrics for main thread scrolling reasons

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 8cc83d3eb0110d32030bb2172d2e5155ca91093f..7d54b6b3ce6c3c7e0c60f9bd4c75ddacf2a3e5db 100644
--- a/cc/input/main_thread_scrolling_reason.h
+++ b/cc/input/main_thread_scrolling_reason.h
@@ -31,6 +31,7 @@ struct MainThreadScrollingReason {
     // 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,
@@ -42,7 +43,7 @@ struct MainThreadScrollingReason {
     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
@@ -52,7 +53,7 @@ struct MainThreadScrollingReason {
         kNotScrollingOnMain | kHasBackgroundAttachmentFixedObjects |
         kHasNonLayerViewportConstrainedObjects | kThreadedScrollingDisabled |
         kScrollbarScrolling | kPageOverlay | kHandlingScrollFromMainThread |
-        kCustomScrollbarScrolling;
+        kCustomScrollbarScrolling | kHasOpacity;
     return (reasons & reasons_set_by_main_thread) == reasons;
   }
 
@@ -97,6 +98,8 @@ struct MainThreadScrollingReason {
       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)