Add onIsEncoded to image decoders.

third_party/WebKit/Source/platform/graphics/DecodingImageGenerator.cpp

Index: third_party/WebKit/Source/platform/graphics/DecodingImageGenerator.cpp
diff --git a/third_party/WebKit/Source/platform/graphics/DecodingImageGenerator.cpp b/third_party/WebKit/Source/platform/graphics/DecodingImageGenerator.cpp
index 4f5baec3aaca935e218d3efc0bd7f9a64304d510..a05cb9ae5de1f2920dfed88bf8dd9506412f8596 100644
--- a/third_party/WebKit/Source/platform/graphics/DecodingImageGenerator.cpp
+++ b/third_party/WebKit/Source/platform/graphics/DecodingImageGenerator.cpp
@@ -52,6 +52,10 @@ DecodingImageGenerator::DecodingImageGenerator(
 
 DecodingImageGenerator::~DecodingImageGenerator() {}
 
+bool DecodingImageGenerator::onIsEncoded() const {
+  return true;
+}
+
 SkData* DecodingImageGenerator::onRefEncodedData(GrContext* ctx) {
   TRACE_EVENT0("blink", "DecodingImageGenerator::refEncodedData");