Work around PanGestureRecognizer iOS10 issue.

ios/chrome/browser/ui/side_swipe_gesture_recognizer.mm

 // Copyright 2012 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/side_swipe_gesture_recognizer.h"
 
 #include <cmath>
 
 #include "base/logging.h"
 
@@ skipping 29 matching lines @@
 // immediately if the touches aren't at the edge of the touched view.
 - (void)touchesBegan:(NSSet*)touches withEvent:(UIEvent*)event {
   [super touchesBegan:touches withEvent:event];
 
   // Don't interrupt gestures in progress.
   if (self.state != UIGestureRecognizerStatePossible)
     return;
 
   UITouch* touch = [[event allTouches] anyObject];
   CGPoint location = [touch locationInView:self.view];
+  _startPoint = CGPointZero;
   if (_swipeEdge > 0) {
     if (location.x > _swipeEdge &&
         location.x < CGRectGetMaxX([self.view bounds]) - _swipeEdge) {
       self.state = UIGestureRecognizerStateFailed;
     } else {
       if (location.x < _swipeEdge) {
         _direction = UISwipeGestureRecognizerDirectionRight;
       } else {
         _direction = UISwipeGestureRecognizerDirectionLeft;
       }
@@ skipping 11 matching lines @@
     [super touchesMoved:touches withEvent:event];
     return;
   }
 
   // Only one touch.
   if ([[event allTouches] count] > 1) {
     self.state = UIGestureRecognizerStateFailed;
     return;
   }
 
+  // In iOS10, sometimes a PanGestureRecognizer will fire a touchesMoved even
+  // after touchesBegan sets its state to |UIGestureRecognizerStateFailed|.
+  // Somehow the state is re-set to UIGestureRecognizerStatePossible, and ends
+  // up in moved.  Checking if |_startPoint| has been set is a secondary way to
+  // catch for failed gestures.
+  if (CGPointEqualToPoint(_startPoint, CGPointZero)) {
+    self.state = UIGestureRecognizerStateFailed;
+    return;
+  }
+
   // Don't swipe at an angle greater than |kMaxSwipeYAngle|.
   UITouch* touch = [[event allTouches] anyObject];
   CGPoint currentPoint = [touch locationInView:self.view];
   CGFloat dy = currentPoint.y - _startPoint.y;
   CGFloat dx = std::abs(currentPoint.x - _startPoint.x);
   CGFloat degrees = std::fabs(std::atan2(dy, dx) * 180 / CGFloat(M_PI));
   if (degrees > kMaxSwipeYAngle) {
     self.state = UIGestureRecognizerStateFailed;
     return;
   }
@@ skipping 39 matching lines @@
   _startPoint = CGPointZero;
   [super touchesEnded:touches withEvent:event];
 }
 
 - (void)touchesCancelled:(NSSet*)touches withEvent:(UIEvent*)event {
   _startPoint = CGPointZero;
   [super touchesCancelled:touches withEvent:event];
 }
 
 @end