Make SkImageFilter methods const.

src/effects/SkDisplacementMapEffect.cpp

Index: src/effects/SkDisplacementMapEffect.cpp
diff --git a/src/effects/SkDisplacementMapEffect.cpp b/src/effects/SkDisplacementMapEffect.cpp
index 555f795a519c36ff8d4414a614e174b337bf5865..a563a6ef8593ab9482a96a8fb1c9a396cd0b823b 100644
--- a/src/effects/SkDisplacementMapEffect.cpp
+++ b/src/effects/SkDisplacementMapEffect.cpp
@@ -195,10 +195,10 @@ bool SkDisplacementMapEffect::onFilterImage(Proxy* proxy,
                                             const SkBitmap& src,
                                             const SkMatrix& ctm,
                                             SkBitmap* dst,
-                                            SkIPoint* offset) {
+                                            SkIPoint* offset) const {
     SkBitmap displ = src, color = src;
-    SkImageFilter* colorInput = getColorInput();
-    SkImageFilter* displInput = getDisplacementInput();
+    const SkImageFilter* colorInput = getColorInput();
+    const SkImageFilter* displInput = getDisplacementInput();
     SkIPoint colorOffset = SkIPoint::Make(0, 0), displOffset = SkIPoint::Make(0, 0);
     if ((colorInput && !colorInput->filterImage(proxy, src, ctm, &color, &colorOffset)) ||
         (displInput && !displInput->filterImage(proxy, src, ctm, &displ, &displOffset))) {
@@ -348,7 +348,7 @@ private:
 };
 
 bool SkDisplacementMapEffect::filterImageGPU(Proxy* proxy, const SkBitmap& src, const SkMatrix& ctm,
-                                             SkBitmap* result, SkIPoint* offset) {
+                                             SkBitmap* result, SkIPoint* offset) const {
     SkBitmap colorBM;
     SkIPoint colorOffset = SkIPoint::Make(0, 0);
     if (!SkImageFilterUtils::GetInputResultGPU(getColorInput(), proxy, src, ctm, &colorBM,