Fix the crash that could occur when the window is closed while web contents drag is in progress.

content/browser/web_contents/web_drag_source_mac.h

 // Copyright (c) 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 <Cocoa/Cocoa.h>
 
 #include "base/file_path.h"
 #include "base/memory/scoped_nsobject.h"
 #include "base/memory/scoped_ptr.h"
 #include "googleurl/src/gurl.h"
@@ skipping 40 matching lines @@
 // contentsView and with the given dropData and pboard). Fill the pasteboard
 // with data types appropriate for dropData.
 - (id)initWithContents:(WebContentsImpl*)contents
                   view:(NSView*)contentsView
               dropData:(const WebDropData*)dropData
                  image:(NSImage*)image
                 offset:(NSPoint)offset
             pasteboard:(NSPasteboard*)pboard
      dragOperationMask:(NSDragOperation)dragOperationMask;
 
+// Call when the web contents is gone.
+- (void)clearWebContentsView;
+
 // Returns a mask of the allowed drag operations.
 - (NSDragOperation)draggingSourceOperationMaskForLocal:(BOOL)isLocal;
 
 // Call when asked to do a lazy write to the pasteboard; hook up to
 // -pasteboard:provideDataForType: (on the contentsView).
 - (void)lazyWriteToPasteboard:(NSPasteboard*)pboard
                       forType:(NSString*)type;
 
 // Start the drag (on the originally provided contentsView); can do this right
 // after -initWithContentsView:....
 - (void)startDrag;
 
 // End the drag and clear the pasteboard; hook up to
 // -draggedImage:endedAt:operation:.
 - (void)endDragAt:(NSPoint)screenPoint
         operation:(NSDragOperation)operation;
 
 // Drag moved; hook up to -draggedImage:movedTo:.
 - (void)moveDragTo:(NSPoint)screenPoint;
 
 // Call to drag a promised file to the given path (should be called before
 // -endDragAt:...); hook up to -namesOfPromisedFilesDroppedAtDestination:.
 // Returns the file name (not including path) of the file deposited (or which
 // will be deposited).
 - (NSString*)dragPromisedFileTo:(NSString*)path;
 
 @end