Bug 1071514 - [Woodduck][Messages] The picture will flicker once when yo...

...u take one picture as attachment r=schung
Once you used a Blob URL instead of inserting using innerHTML, it's more likely to missing the contentNode then sizeIndicator(unlike the previous mechanism, if the contentNode exists, so does sizeIndicator since the template is created at first). I did reproduce "once" that contentNode missing, and the image resize checking will never finish if we get exception here. Maybe we can simply check contentNode for now, and create another bug for this edge case(really rare, but not sure how it performs on low-end device). How about change getContentNode into promise with attachmentContainer load event listener?