Make SkImageFilter methods const.

src/effects/SkBlurImageFilter.cpp

Index: src/effects/SkBlurImageFilter.cpp
diff --git a/src/effects/SkBlurImageFilter.cpp b/src/effects/SkBlurImageFilter.cpp
index 67b0511480eaff63c99272f722c2bb2951f71e3a..a08b9c56754934604ccad92d350bc71f488cbab0 100644
--- a/src/effects/SkBlurImageFilter.cpp
+++ b/src/effects/SkBlurImageFilter.cpp
@@ -136,7 +136,7 @@ static void getBox3Params(SkScalar s, int *kernelSize, int* kernelSize3, int *lo
 
 bool SkBlurImageFilter::onFilterImage(Proxy* proxy,
                                       const SkBitmap& source, const SkMatrix& ctm,
-                                      SkBitmap* dst, SkIPoint* offset) {
+                                      SkBitmap* dst, SkIPoint* offset) const {
     SkBitmap src = source;
     SkIPoint srcOffset = SkIPoint::Make(0, 0);
     if (getInput(0) && !getInput(0)->filterImage(proxy, source, ctm, &src, &srcOffset)) {
@@ -252,7 +252,7 @@ bool SkBlurImageFilter::onFilterBounds(const SkIRect& src, const SkMatrix& ctm,
 }
 
 bool SkBlurImageFilter::filterImageGPU(Proxy* proxy, const SkBitmap& src, const SkMatrix& ctm,
-                                       SkBitmap* result, SkIPoint* offset) {
+                                       SkBitmap* result, SkIPoint* offset) const {
 #if SK_SUPPORT_GPU
     SkBitmap input;
     SkIPoint srcOffset = SkIPoint::Make(0, 0);