Add support for touch-action: pinch-zoom.

content/browser/renderer_host/input/touch_action_filter.cc

Index: content/browser/renderer_host/input/touch_action_filter.cc
diff --git a/content/browser/renderer_host/input/touch_action_filter.cc b/content/browser/renderer_host/input/touch_action_filter.cc
index e8f23354bd6a27e205fc3d3cad77030a70268ff8..e2a8e69e863a1517a13076ef37dbec127eaebf5b 100644
--- a/content/browser/renderer_host/input/touch_action_filter.cc
+++ b/content/browser/renderer_host/input/touch_action_filter.cc
@@ -88,16 +88,8 @@ bool TouchActionFilter::FilterGestureEvent(WebGestureEvent* gesture_event) {
 
     case WebInputEvent::GesturePinchBegin:
       DCHECK(!drop_pinch_gesture_events_);
-      if (allowed_touch_action_ & TOUCH_ACTION_PINCH_ZOOM) {
-        // Pinch events are always bracketed by scroll events, and the W3C

[Rick Byers, 2016/07/07 21:09:14]

Ah crap, I forgot about this comment I left.

I think this change may break some things.  IIRC we may depend on
GesturePinchUpdate occuring inside of a GestureScroll sequence (pinch events
outside of a scroll sequence were impossible before AFAIK).

Also if we're actually inside of a pinch sequence, then we probably want to
allow scroll events so that two-finger panning during a pinch still works in a
'touch-action: pinch-zoom' region.  Let's chat about options here.

-        // standard touch-action provides no way to disable scrolling without
-        // also disabling pinching (validated by the IPC ENUM traits).
-        DCHECK(allowed_touch_action_ == TOUCH_ACTION_AUTO ||
-            allowed_touch_action_ == TOUCH_ACTION_MANIPULATION);
-        DCHECK(!drop_scroll_gesture_events_);
-      } else {
-        drop_pinch_gesture_events_ = true;
-      }
+      drop_pinch_gesture_events_ =
+          (allowed_touch_action_ & TOUCH_ACTION_PINCH_ZOOM) != 0;
       return drop_pinch_gesture_events_;
 
     case WebInputEvent::GesturePinchUpdate:
@@ -108,7 +100,6 @@ bool TouchActionFilter::FilterGestureEvent(WebGestureEvent* gesture_event) {
         drop_pinch_gesture_events_ = false;
         return true;
       }
-      DCHECK(!drop_scroll_gesture_events_);
       break;
 
     // The double tap gesture is a tap ending event. If a double tap gesture is