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

third_party/WebKit/public/web/WebInputEvent.h

Index: third_party/WebKit/public/web/WebInputEvent.h
diff --git a/third_party/WebKit/public/web/WebInputEvent.h b/third_party/WebKit/public/web/WebInputEvent.h
index 63837299dabd7772bda81521075d9656d6c3f11a..e9ac846d6494329be5afa2367a23f3e4dec02548 100644
--- a/third_party/WebKit/public/web/WebInputEvent.h
+++ b/third_party/WebKit/public/web/WebInputEvent.h
@@ -633,6 +633,9 @@ public:
     // 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;
 
@@ -641,6 +644,7 @@ public:
         , touchesLength(0)
         , dispatchType(Blocking)
         , movedBeyondSlopRegion(false)
+        , shouldForceBePassive(false)
         , uniqueTouchEventId(0)
     {
     }