Add raw pixel serialization fallback for SkImages that cannot be encoded.

src/core/SkWriteBuffer.cpp

Index: src/core/SkWriteBuffer.cpp
diff --git a/src/core/SkWriteBuffer.cpp b/src/core/SkWriteBuffer.cpp
index 060bd43c6f2697e8f9583e8f4f2e016dc92236cc..ccf1f37dd8fb890593665f177f248949218a38f7 100644
--- a/src/core/SkWriteBuffer.cpp
+++ b/src/core/SkWriteBuffer.cpp
@@ -182,6 +182,13 @@ void SkBinaryWriteBuffer::writeImage(const SkImage* image) {
         return;
     }
 
+    SkBitmap bm;
+    if (image->asLegacyBitmap(&bm, SkImage::kRO_LegacyBitmapMode)) {
+        this->writeUInt(1);  // signal raw pixels.
+        SkBitmap::WriteRawPixels(this, bm);
+        return;
+    }
+
     this->writeUInt(0); // signal no pixels (in place of the size of the encoded data)
 }