Added code to disable 'paste' if nothing is present in clipboard to paste.

Source/core/editing/Editor.cpp

 /*
  * Copyright (C) 2006, 2007, 2008, 2011 Apple Inc. All rights reserved.
  * Copyright (C) 2008 Nokia Corporation and/or its subsidiary(-ies)
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  * 1. Redistributions of source code must retain the above copyright
  *    notice, this list of conditions and the following disclaimer.
  * 2. Redistributions in binary form must reproduce the above copyright
@@ skipping 221 matching lines @@
 bool Editor::canCopy() const
 {
     if (imageElementFromImageDocument(m_frame.document()))
         return true;
     FrameSelection& selection = m_frame.selection();
     return selection.isRange() && !selection.isInPasswordField();
 }
 
 bool Editor::canPaste() const
 {
-    return canEdit();
+    if (!canEdit())
+        return false;
+    String text = Pasteboard::generalPasteboard()->plainText();

[jamesr, 2014/07/24 18:27:15]

this is going to do a synchronous IPC to the browser process on every call,
fetch the entire paste buffer, and copy it back to the renderer just to throw
everything away but 1 bit.  This function appears to be called pretty often.  Is
there no better way to do this?

+    return !text.isEmpty();
 }
 
 bool Editor::canDelete() const
 {
     FrameSelection& selection = m_frame.selection();
     return selection.isRange() && selection.rootEditableElement();
 }
 
 bool Editor::canDeleteRange(Range* range) const
 {
@@ skipping 1025 matching lines @@
     frame().selection().setShouldShowBlockCursor(m_overwriteModeEnabled);
 }
 
 void Editor::trace(Visitor* visitor)
 {
     visitor->trace(m_lastEditCommand);
     visitor->trace(m_mark);
 }
 
 } // namespace blink