Make SkImageFilter methods const.

src/gpu/SkGpuDevice.cpp

Index: src/gpu/SkGpuDevice.cpp
diff --git a/src/gpu/SkGpuDevice.cpp b/src/gpu/SkGpuDevice.cpp
index 8c272f947abc60fa3d648d9a1459a33531c61e1a..a3af3c3d08ca753d544e3ba93685382a2cfd36c9 100644
--- a/src/gpu/SkGpuDevice.cpp
+++ b/src/gpu/SkGpuDevice.cpp
@@ -1515,7 +1515,7 @@ void SkGpuDevice::internalDrawBitmap(const SkBitmap& bitmap,
 }
 
 static bool filter_texture(SkBaseDevice* device, GrContext* context,
-                           GrTexture* texture, SkImageFilter* filter,
+                           GrTexture* texture, const SkImageFilter* filter,
                            int w, int h, const SkMatrix& ctm, SkBitmap* result,
                            SkIPoint* offset) {
     SkASSERT(filter);
@@ -1694,11 +1694,11 @@ void SkGpuDevice::drawDevice(const SkDraw& draw, SkBaseDevice* device,
     fContext->drawRectToRect(grPaint, dstRect, srcRect);
 }
 
-bool SkGpuDevice::canHandleImageFilter(SkImageFilter* filter) {
+bool SkGpuDevice::canHandleImageFilter(const SkImageFilter* filter) {
     return filter->canFilterImageGPU();
 }
 
-bool SkGpuDevice::filterImage(SkImageFilter* filter, const SkBitmap& src,
+bool SkGpuDevice::filterImage(const SkImageFilter* filter, const SkBitmap& src,
                               const SkMatrix& ctm,
                               SkBitmap* result, SkIPoint* offset) {
     // want explicitly our impl, so guard against a subclass of us overriding it