bindings: Add a macro for V8 Maybe<T> APIs.

Source/bindings/core/v8/V8BindingMacros.h

 /*
  * Copyright (C) 2010 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 59 matching lines @@
 #define TOSTRING_VOID(type, var, value) \
     type var(value);                    \
     if (UNLIKELY(!var.prepare()))       \
         return;
 
 #define TOSTRING_DEFAULT(type, var, value, retVal) \
     type var(value);                               \
     if (UNLIKELY(!var.prepare()))                  \
         return retVal;
 
+template <typename T>
+inline bool getValueFromMaybe(v8::Maybe<T> maybe, T& outVariable)
+{
+    if (maybe.IsNothing())
+        return false;
+    outVariable = maybe.FromJust();
+    return true;
+}
+
+// The last "else" is to avoid dangling else problem.
+#define V8_CALL(outVariable, handle, methodCall, failureExpression)                \
+    if (handle.IsEmpty() || !getValueFromMaybe(handle->methodCall, outVariable)) { \
+        failureExpression;                                                         \
+    } else
+
 } // namespace blink
 
 #endif // V8BindingMacros_h