Add UMA metric for tracking listeners for blocking touch while fling is happening

third_party/WebKit/public/web/WebInputEvent.h

 /*
  * Copyright (C) 2009 Google Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions are
  * met:
  *
  *     * Redistributions of source code must retain the above copyright
  * notice, this list of conditions and the following disclaimer.
  *     * Redistributions in binary form must reproduce the above
@@ skipping 615 matching lines @@
 
     // Whether the event is blocking, non-blocking, all event
     // listeners were passive or was forced to be non-blocking.
     DispatchType dispatchType;
 
     // For a single touch, this is true after the touch-point has moved beyond
     // the platform slop region. For a multitouch, this is true after any
     // touch-point has moved (by whatever amount).
     bool movedBeyondSlopRegion;
 
+    // We will force the touch starts to be passive when during an ative fling animation.
+    bool shouldForceBePassive;

[dtapuska, 2016/05/05 20:22:21]

Combining this change with https://codereview.chromium.org/1955643002/

indicates to me that we actually have an instance where state variables don't
actually make sense.

For example consider this object:
dispatchType == ListenersForcedNonBlockingPassive
shouldForceBePassive == false

That is an incorrect state. I wonder if we should instead add a different
enumeration value on the DispatchType indicated
BlockingCandidateForForcedNonBlocking

which is a bit indicating; hey this is still a blocking event; but we want to
tag it for metrics purposes that it might be forced non-blocking under a
heuristic.

Thoughts?

[tdresser, 2016/05/06 19:01:31]

Let's just call this "dispatchedDuringFling", and we can rename it later if
necessary.

That get's rid of the case where we have state that doesn't make sense.

+
     // A unique identifier for the touch event.
     uint32_t uniqueTouchEventId;
 
     WebTouchEvent()
         : WebInputEvent(sizeof(WebTouchEvent))
         , touchesLength(0)
         , dispatchType(Blocking)
         , movedBeyondSlopRegion(false)
+        , shouldForceBePassive(false)
         , uniqueTouchEventId(0)
     {
     }
 };
 
 #pragma pack(pop)
 
 } // namespace blink
 
 #endif