Enable multiple touch in WebView.

ios/chrome/browser/ui/overscroll_actions/overscroll_actions_controller.mm

 // Copyright 2015 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #import "ios/chrome/browser/ui/overscroll_actions/overscroll_actions_controller.h"
 
 #import <QuartzCore/QuartzCore.h>
 
 #include <algorithm>
 #include "base/logging.h"
@@ skipping 461 matching lines @@
                             withVelocity:(CGPoint)velocity
                      targetContentOffset:(inout CGPoint*)targetContentOffset {
   DCHECK_EQ(static_cast<id>(webViewScrollViewProxy), [self scrollView]);
   [self scrollViewWillEndDraggingWithVelocity:velocity
                           targetContentOffset:targetContentOffset];
 }
 
 #pragma mark - Pan gesture recognizer handling
 
 - (void)panGesture:(UIPanGestureRecognizer*)gesture {
+  if (gesture.state == UIGestureRecognizerStateEnded ||
+      gesture.state == UIGestureRecognizerStateCancelled) {
+    [self setWebViewInteractionEnabled:YES];
+  }
+  if (self.overscrollState == OverscrollState::NO_PULL_STARTED) {
+    return;
+  }
+
   if (gesture.state == UIGestureRecognizerStateBegan) {
     [self setWebViewInteractionEnabled:NO];
-  } else if (gesture.state == UIGestureRecognizerStateEnded ||
-             gesture.state == UIGestureRecognizerStateCancelled) {
-    [self setWebViewInteractionEnabled:YES];
   }
+
   const CGPoint panPointScreen = [gesture locationInView:nil];
   if (self.overscrollState == OverscrollState::ACTION_READY) {
     const CGFloat direction = UseRTLLayout() ? -1 : 1;
     const CGFloat xOffset = direction *
                             (panPointScreen.x - self.panPointScreenOrigin.x) /
                             kHorizontalPanDistance;
 
     [self.overscrollActionView updateWithHorizontalOffset:xOffset];
   }
 }
@@ skipping 413 matching lines @@
       overscrollActionsController:self
                  didTriggerAction:self.overscrollActionView.selectedAction];
 }
 
 - (void)overscrollActionsView:(OverscrollActionsView*)view
       selectedActionDidChange:(OverscrollAction)newAction {
   TriggerHapticFeedbackForSelectionChange();
 }
 
 @end