Make SkImageFilter methods const.

src/pdf/SkPDFDevice.cpp

 /*
  * Copyright 2011 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #include "SkPDFDevice.h"
 
 #include "SkAnnotation.h"
@@ skipping 2312 matching lines @@
 
     SkPDFUtils::DrawFormXObject(this->addXObjectResource(image.get()),
                                 &content.entry()->fContent);
 }
 
 bool SkPDFDevice::onReadPixels(const SkBitmap& bitmap, int x, int y,
                                SkCanvas::Config8888) {
     return false;
 }
 
-bool SkPDFDevice::allowImageFilter(SkImageFilter*) {
+bool SkPDFDevice::allowImageFilter(const SkImageFilter*) {
     return false;
 }