Bug 1192733: fix the MediaFormatReader can not back from dormant state. r=jya

--HG--
extra : rebase_source : 185e32c16727be0a5138b936e56c72b20fa03f19

dom/media/MediaFormatReader.cpp

@@ -280,7 +280,12 @@ MediaFormatReader::AsyncReadMetadata()
       return p;
     }
     MOZ_ASSERT(!mDecodersInitRequest.Exists());
-    EnsureDecodersInitialized();
+    if (EnsureDecodersInitialized()) {
+      nsRefPtr<MetadataHolder> metadata = new MetadataHolder();
+      metadata->mInfo = mInfo;
+      metadata->mTags = nullptr;
+      mMetadataPromise.Resolve(metadata, __func__);
+    }
     return p;
   }