Make sure we don't deref nullopt for TouchAction.

This CL is in addition to
https://chromium-review.googlesource.com/c/chromium/src/+/1099193, and
fixes another case where an Optional<TouchAction> might have novalue.

Bug: 824774
Change-Id: I21d18fa682d3de4cea7b54572f0bbef92eb9e6ca
Reviewed-on: https://chromium-review.googlesource.com/1101478Reviewed-by: Ken Buchanan <kenrb@chromium.org>
Reviewed-by: Xida Chen <xidachen@chromium.org>
Commit-Queue: James MacLean <wjmaclean@chromium.org>
Cr-Commit-Position: refs/heads/master@{#567410}

content/browser/renderer_host/render_widget_host_input_event_router.cc

@@ -1105,7 +1105,8 @@ void RenderWidgetHostInputEventRouter::DispatchTouchscreenGestureEvent(
     // should have been one.
     if (root_view != touchscreen_gesture_target_.target &&
         !rwhi->is_in_touchscreen_gesture_scroll()) {
-      base::Optional<cc::TouchAction> target_allowed_touch_action;
+      base::Optional<cc::TouchAction> target_allowed_touch_action(
+          cc::kTouchActionNone);
       if (touchscreen_gesture_target_.target) {
         target_allowed_touch_action =
             (static_cast<RenderWidgetHostImpl*>(
@@ -1113,6 +1114,7 @@ void RenderWidgetHostInputEventRouter::DispatchTouchscreenGestureEvent(
                 ->input_router()
                 ->AllowedTouchAction();
       }
+      DCHECK(target_allowed_touch_action.has_value());
       if (target_allowed_touch_action.value() &
           cc::TouchAction::kTouchActionPinchZoom) {
         gesture_pinch_did_send_scroll_begin_ = true;