Fix crash in ui::ClipboardMac::ReadAvailableTypes.

ui/base/clipboard/clipboard_mac.mm

 // 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.
 
 #include "ui/base/clipboard/clipboard_mac.h"
 
 #import <Cocoa/Cocoa.h>
 #include <stdint.h>
 
 #include <limits>
@@ skipping 21 matching lines @@
 
 // Tells us if WebKit was the last to write to the pasteboard. There's no
 // actual data associated with this type.
 NSString* const kWebSmartPastePboardType = @"NeXT smart paste pasteboard type";
 
 // Pepper custom data format type.
 NSString* const kPepperCustomDataPboardType =
     @"org.chromium.pepper-custom-data";
 
 NSPasteboard* GetPasteboard() {
-  // The pasteboard should not be nil in a UI session, but this handy DCHECK
-  // can help track down problems if someone tries using clipboard code outside
-  // of a UI session.
+  // The pasteboard can always be nil, since there is a finite amount of storage
+  // that must be shared between all pasteboards.
   NSPasteboard* pasteboard = [NSPasteboard generalPasteboard];
-  DCHECK(pasteboard);
   return pasteboard;
 }
 
 }  // namespace
 
 // Clipboard::FormatType implementation.
 Clipboard::FormatType::FormatType() : data_(nil) {
 }
 
 Clipboard::FormatType::FormatType(NSString* native_format)
@@ skipping 163 matching lines @@
                                       std::vector<base::string16>* types,
                                       bool* contains_filenames) const {
   DCHECK(CalledOnValidThread());
   types->clear();
   if (IsFormatAvailable(Clipboard::GetPlainTextFormatType(), type))
     types->push_back(base::UTF8ToUTF16(kMimeTypeText));
   if (IsFormatAvailable(Clipboard::GetHtmlFormatType(), type))
     types->push_back(base::UTF8ToUTF16(kMimeTypeHTML));
   if (IsFormatAvailable(Clipboard::GetRtfFormatType(), type))
     types->push_back(base::UTF8ToUTF16(kMimeTypeRTF));
-  if ([NSImage canInitWithPasteboard:GetPasteboard()])
+
+  NSPasteboard* pb = GetPasteboard();
+  if (pb && [NSImage canInitWithPasteboard:pb])
     types->push_back(base::UTF8ToUTF16(kMimeTypePNG));
   *contains_filenames = false;
 
-  NSPasteboard* pb = GetPasteboard();
   if ([[pb types] containsObject:kWebCustomDataPboardType]) {
     NSData* data = [pb dataForType:kWebCustomDataPboardType];
     if ([data length])
       ReadCustomDataTypes([data bytes], [data length], types);
   }
 }
 
 void ClipboardMac::ReadText(ClipboardType type, base::string16* result) const {
   DCHECK(CalledOnValidThread());
   DCHECK_EQ(type, CLIPBOARD_TYPE_COPY_PASTE);
@@ skipping 68 matching lines @@
       // -[NSImage initWithPasteboard:] gets confused with copies of a single
       // file from the Finder, so extract the path ourselves.
       // http://crbug.com/553686
       NSArray* paths = [pb propertyListForType:NSFilenamesPboardType];
       if ([paths count]) {
         // If N number of files are selected from finder, choose the last one.
         image.reset([[NSImage alloc]
             initWithContentsOfURL:[NSURL fileURLWithPath:[paths lastObject]]]);
       }
     } else {
-      image.reset([[NSImage alloc] initWithPasteboard:pb]);
+      if (pb)
+        image.reset([[NSImage alloc] initWithPasteboard:pb]);
     }
   } @catch (id exception) {
   }
 
   SkBitmap bitmap;
   if (image.get()) {
     bitmap = skia::NSImageToSkBitmapWithColorSpace(
         image.get(), /*is_opaque=*/ false, base::mac::GetSystemColorSpace());
   }
   return bitmap;
@@ skipping 121 matching lines @@
 // Write an extra flavor that signifies WebKit was the last to modify the
 // pasteboard. This flavor has no data.
 void ClipboardMac::WriteWebSmartPaste() {
   NSPasteboard* pb = GetPasteboard();
   NSString* format = GetWebKitSmartPasteFormatType().ToNSString();
   [pb addTypes:[NSArray arrayWithObject:format] owner:nil];
   [pb setData:nil forType:format];
 }
 
 }  // namespace ui