Work around PanGestureRecognizer iOS10 issue.

ios/chrome/browser/ui/side_swipe_gesture_recognizer.mm

Index: ios/chrome/browser/ui/side_swipe_gesture_recognizer.mm
diff --git a/ios/chrome/browser/ui/side_swipe_gesture_recognizer.mm b/ios/chrome/browser/ui/side_swipe_gesture_recognizer.mm
index 72f608c76ab89627b019f2e872de700a1cd1958f..b4a463a9db0983458cd7896a51bb9c76617e3f58 100644
--- a/ios/chrome/browser/ui/side_swipe_gesture_recognizer.mm
+++ b/ios/chrome/browser/ui/side_swipe_gesture_recognizer.mm
@@ -47,6 +47,7 @@ const CGFloat kDefaultMinSwipeXThreshold = 4;
 
   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) {
@@ -78,6 +79,16 @@ const CGFloat kDefaultMinSwipeXThreshold = 4;
     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];