Bug 1400537 - P1. Don't assume that the PDM will properly report an error. r=alwu

Only the Windows H264 decoder supports CreateDecoderParam::mError, all the other PDM leave the value untouched.

As such, it can't be assumed that in case of failure, the mError attribute will be set.

MozReview-Commit-ID: GWHGP6Wv3fl

--HG--
extra : rebase_source : 081b71c7a53c41d9a13904e4182e3cfdb876ae43

dom/media/platforms/wrappers/H264Converter.cpp

@@ -267,6 +267,7 @@ H264Converter::CreateDecoder(const VideoInfo& aConfig,
                        RESULT_DETAIL("Invalid SPS NAL."));
   }
 
+  MediaResult error = NS_OK;
   mDecoder = mPDM->CreateVideoDecoder({
     aConfig,
     mTaskQueue,
@@ -277,14 +278,17 @@ H264Converter::CreateDecoder(const VideoInfo& aConfig,
     mType,
     mOnWaitingForKeyEvent,
     mDecoderOptions,
-    &mLastError
+    &error
   });
 
   if (!mDecoder) {
-    MOZ_ASSERT(NS_FAILED(mLastError));
-    return MediaResult(mLastError.Code(),
-                       RESULT_DETAIL("Unable to create H264 decoder, reason = %s.",
-                                     mLastError.Description().get()));
+    if (NS_FAILED(error)) {
+      // The decoder supports CreateDecoderParam::mError, returns the value.
+      return error;
+    } else {
+      return MediaResult(NS_ERROR_DOM_MEDIA_FATAL_ERR,
+                         RESULT_DETAIL("Unable to create H264 decoder"));
+    }
   }
 
   mNeedKeyframe = true;