Implement Asynchronous image encoding for Canvas.toBlob

Source/core/fileapi/File.h

Index: Source/core/fileapi/File.h
diff --git a/Source/core/fileapi/File.h b/Source/core/fileapi/File.h
index a620592fd0b823b8985ea1c820b4053a360ecef0..ed0b4dbc5da4c354ad5e94be2221378425892334 100644
--- a/Source/core/fileapi/File.h
+++ b/Source/core/fileapi/File.h
@@ -68,7 +68,7 @@ public:
         return new File(name, modificationTime, blobDataHandle);
     }
 
-    static File* create(char* data, size_t bytes, const String& mimeType);
+    static File* create(const char* data, size_t bytes, const String& mimeType);
 
     // For deserialization.
     static File* createFromSerialization(const String& path, const String& name, const String& relativePath, UserVisibility userVisibility, bool hasSnaphotData, uint64_t size, double lastModified, PassRefPtr<BlobDataHandle> blobDataHandle)