IDL: Don't lose nullable flag when "preprocessing" type

Source/core/html/canvas/WebGLRenderingContextBase.h

Index: Source/core/html/canvas/WebGLRenderingContextBase.h
diff --git a/Source/core/html/canvas/WebGLRenderingContextBase.h b/Source/core/html/canvas/WebGLRenderingContextBase.h
index 7f6f01e1bf811f9f8d430340c94cfb3fe496543b..ce3d6705690ecf09ff1446249c7d3b69846a70d2 100644
--- a/Source/core/html/canvas/WebGLRenderingContextBase.h
+++ b/Source/core/html/canvas/WebGLRenderingContextBase.h
@@ -199,7 +199,7 @@ public:
     ScriptValue getBufferParameter(ScriptState*, GLenum target, GLenum pname);
     void getContextAttributes(Nullable<WebGLContextAttributes>&);
     GLenum getError();
-    PassRefPtrWillBeRawPtr<WebGLExtension> getExtension(const String& name);

[Jens Widell, 2015/02/16 14:33:54]

This signature worked only by mistake. A method returning "object" is expected
to return ScriptValue, and the binding code will be

  v8SetReturnValue(info, <expression>.v8Value());

which compiles when <expression> is a ScriptValue, but not when it is what this
function returns.

However, this method was declared to return "object?", and there was a bug that
then disabled the conversion from "object" to "ScriptValue" (because the type
was nullable), the binding code ended up being

  v8SetReturnValue(info, <expression>);

which compiles fine when <expression> is what this function returns.

+    ScriptValue getExtension(ScriptState*, const String& name);
     ScriptValue getFramebufferAttachmentParameter(ScriptState*, GLenum target, GLenum attachment, GLenum pname);
     ScriptValue getParameter(ScriptState*, GLenum pname);
     ScriptValue getProgramParameter(ScriptState*, WebGLProgram*, GLenum pname);