bug 1362404 - Handle out-of-order overlapping event coalescing. r=masayuki

There are situations where events may be handled out-of-order. Allow
coalescing to happen backwards in time as well as forwards to cover them.

MozReview-Commit-ID: 3k7vNFxE1cz

--HG--
extra : rebase_source : 636a3671536747cf62cb410836b4c5a2275bbb49

layout/base/PresShell.cpp

@@ -8285,6 +8285,10 @@ PresShell::HandleEventInternal(WidgetEvent* aEvent,
                               lastMillis);
       }
       sLastInputCreated = aEvent->mTimeStamp;
+    } else if (aEvent->mTimeStamp < sLastInputCreated) {
+      // This event was created before the last input. May be processing out
+      // of order, so coalesce backwards, too.
+      sLastInputCreated = aEvent->mTimeStamp;
     }
     sLastInputProcessed = now;
   }