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..191318b60a336982efaff0e489ffcd56245033ce 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,

[pdr., 2017/01/07 22:25:25]

Why does opacity (and later patches: transform) prevent composited scrolling? Is
it due to how opacity and lcd text interact?

[bokan, 2017/01/09 13:29:08]

Right, I think it's because in order to do LCD text antialiasing, you need to
know what's behind the text. Opacity means other layers in CC might change
what's behind the text. Transforms I'm less sure, I'm guessing some transforms
might "ruin" the antialiasing (e.g. perspective might distort the subpixel
adjustments across multiple pixels)? Just to clarify, these patches are just
adding metrics to existing compositing outs rather than actually changing what's
getting composited.

[pdr., 2017/01/09 17:22:12]

Thanks for the explanation. Would you be okay with changing kHasOpacity to
kHasOpacityAndLCDText? I can put up a patch to do that.

Also, aren't we using these flags for real performance changes by forcing the
main thread? If not, why don't we put them on a different structure instead of
re-using MainThreadScrollingReason?

[bokan, 2017/01/09 17:54:56]

Yeah, that sounds clearer. sgtm
For these newly added flags, my understanding is that we wouldn't have created a
cc::Layer in these cases at all so there was nothing to put the reason onto.
We're now storing these on the ScrollableArea as well as the cc::Layer but (on
main thread) it's being used to track stats rather than making compositing
decisions.

 
     // 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)
@@ -115,6 +118,20 @@ 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 = 0;
+    while (reason > 0) {
+      reason = reason >> 1;
+      ++index;
+    }
+    return index;
+  }
 };
 
 }  // namespace cc