Switch HTMLCanvasElement over to new-style ExceptionState.

Source/bindings/v8/ExceptionState.h

 /*
  * Copyright (C) 2013 Google Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions are
  * met:
  *
  *     * Redistributions of source code must retain the above copyright
  * notice, this list of conditions and the following disclaimer.
  *     * Redistributions in binary form must reproduce the above
@@ skipping 70 matching lines @@
     virtual void throwTypeError(const String& message);
     virtual void throwSecurityError(const String& sanitizedMessage, const String& unsanitizedMessage = String());
 
     // Please don't use these methods. Use ::throwDOMException and ::throwTypeError, and pass in a useful exception message.
     virtual void throwUninformativeAndGenericDOMException(const ExceptionCode& ec) { throwDOMException(ec, String()); }
     virtual void throwUninformativeAndGenericTypeError() { throwTypeError(String()); }
 
     bool hadException() const { return !m_exception.isEmpty() || m_code; }
     void clearException();
 
-    ExceptionCode code() { return m_code; }
+    ExceptionCode code() const { return m_code; }
 
     bool throwIfNeeded()
     {
         if (m_exception.isEmpty()) {
             if (!m_code)
                 return false;
             throwUninformativeAndGenericDOMException(m_code);
         }
 
         V8ThrowException::throwError(m_exception.newLocal(m_isolate), m_isolate);
         return true;
     }
 
-    Context context() { return m_context; }
-    const char* propertyName() { return m_propertyName; }
-    const char* interfaceName() { return m_interfaceName; }
+    Context context() const { return m_context; }
+    const char* propertyName() const { return m_propertyName; }
+    const char* interfaceName() const { return m_interfaceName; }
 
 protected:
     ExceptionCode m_code;
     Context m_context;
     const char* m_propertyName;
     const char* m_interfaceName;
 
 private:
     void setException(v8::Handle<v8::Value>);
 
+    String addExceptionContext(const String&) const;
+
     ScopedPersistent<v8::Value> m_exception;
     v8::Handle<v8::Object> m_creationContext;
     v8::Isolate* m_isolate;
 };
 
 class TrackExceptionState : public ExceptionState {
 public:
     TrackExceptionState(): ExceptionState(v8::Handle<v8::Object>(), 0) { }
     virtual void throwDOMException(const ExceptionCode&, const String& message) OVERRIDE FINAL;
     virtual void throwTypeError(const String& message = String()) OVERRIDE FINAL;
     virtual void throwSecurityError(const String& sanitizedMessage, const String& unsanitizedMessage = String()) OVERRIDE FINAL;
 };
 
 } // namespace WebCore
 
 #endif // ExceptionState_h