Implement a persistent uniqueID-based cache for SkImageFilter.

include/core/SkPicture.h

Index: include/core/SkPicture.h
diff --git a/include/core/SkPicture.h b/include/core/SkPicture.h
index 21ebef32cd661c17c690a9dabe69b1107e03f052..c733e532c291bb64bb5eb52c42991d4ae57e56d1 100644
--- a/include/core/SkPicture.h
+++ b/include/core/SkPicture.h
@@ -230,13 +230,14 @@ private:
     // V28: No longer call bitmap::flatten inside SkWriteBuffer::writeBitmap.
     // V29: Removed SaveFlags parameter from save().
     // V30: Remove redundant SkMatrix from SkLocalMatrixShader.
+    // V31: Add a serialized UniqueID to SkImageFilter.
 
     // Note: If the picture version needs to be increased then please follow the
     // steps to generate new SKPs in (only accessible to Googlers): http://goo.gl/qATVcw
 
     // Only SKPs within the min/current picture version range (inclusive) can be read.
     static const uint32_t MIN_PICTURE_VERSION = 19;
-    static const uint32_t CURRENT_PICTURE_VERSION = 30;
+    static const uint32_t CURRENT_PICTURE_VERSION = 31;
 
     mutable uint32_t      fUniqueID;