Make SkImageFilter methods const.

include/core/SkBitmapDevice.h

Index: include/core/SkBitmapDevice.h
diff --git a/include/core/SkBitmapDevice.h b/include/core/SkBitmapDevice.h
index 945ffbfca474271544d6a9444feb64bbd119064d..e4f9b88a5e68b9f6c60adff3b0604d90a17039c2 100644
--- a/include/core/SkBitmapDevice.h
+++ b/include/core/SkBitmapDevice.h
@@ -227,7 +227,7 @@ protected:
      *  some subclasses that do not support pixel manipulations after drawing
      *  has occurred (e.g. printing). The default implementation returns true.
      */
-    virtual bool allowImageFilter(SkImageFilter*) SK_OVERRIDE;
+    virtual bool allowImageFilter(const SkImageFilter*) SK_OVERRIDE;
 
     /**
      *  Override and return true for filters that the device can handle
@@ -236,7 +236,7 @@ protected:
      *  Returning false means the SkCanvas will have apply the filter itself,
      *  and just pass the resulting image to the device.
      */
-    virtual bool canHandleImageFilter(SkImageFilter*) SK_OVERRIDE;
+    virtual bool canHandleImageFilter(const SkImageFilter*) SK_OVERRIDE;
 
     /**
      *  Related (but not required) to canHandleImageFilter, this method returns
@@ -245,7 +245,7 @@ protected:
      *  If the device does not recognize or support this filter,
      *  it just returns false and leaves result and offset unchanged.
      */
-    virtual bool filterImage(SkImageFilter*, const SkBitmap&, const SkMatrix&,
+    virtual bool filterImage(const SkImageFilter*, const SkBitmap&, const SkMatrix&,
                              SkBitmap* result, SkIPoint* offset) SK_OVERRIDE;
 
 private: