Bug 1332160 - use nsCString internally whenever possible. r=gerald

MozReview-Commit-ID: 1DUaJWfbL88

dom/html/HTMLMediaElement.cpp

@@ -1447,7 +1447,9 @@ void
 HTMLMediaElement::GetMozDebugReaderData(nsAString& aString)
 {
   if (mDecoder && !mSrcStream) {
-    mDecoder->GetMozDebugReaderData(aString);
+    nsAutoCString result;
+    mDecoder->GetMozDebugReaderData(result);
+    aString = NS_ConvertUTF8toUTF16(result);
   }
 }