add ImageGenerator::NewFromData to porting layer

src/lazy/SkDiscardablePixelRef.cpp

Index: src/lazy/SkDiscardablePixelRef.cpp
diff --git a/src/lazy/SkDiscardablePixelRef.cpp b/src/lazy/SkDiscardablePixelRef.cpp
index 84152ed188a4d32ec2c58f74289da55396b46bda..4b7d47b94d2bb4068f0dfcfbe66b3f752a294b88 100644
--- a/src/lazy/SkDiscardablePixelRef.cpp
+++ b/src/lazy/SkDiscardablePixelRef.cpp
@@ -116,7 +116,13 @@ bool SkInstallDiscardablePixelRef(SkImageGenerator* generator, SkBitmap* dst,
     return true;
 }
 
-// This is the public API
+// These are the public API
+
 bool SkInstallDiscardablePixelRef(SkImageGenerator* generator, SkBitmap* dst) {
     return SkInstallDiscardablePixelRef(generator, dst, NULL);
 }
+
+bool SkInstallDiscardablePixelRef(SkData* encoded, SkBitmap* dst) {
+    SkImageGenerator* generator = SkImageGenerator::NewFromData(encoded);
+    return generator ? SkInstallDiscardablePixelRef(generator, dst, NULL) : false;
+}