While reviewing a related CL I found two potential issues with the MHTMLExtraParts implementation. They might not reflect into actual problems -- or might even be WAI -- but I wanted to make sure they were known:

* MHTMLExtraParts data is never cleared from the instance held by WebContents. If a second MHTML save happens using the same WebContents instance new data will be appended along with what was already there and that later save would store both the old and new data.

* A single instance is shared among all potential users of that same WebContents. This would be a problem in the less likely (but possible) case of two MHTML save operations happening in parallel, causing more extra data to be added and then saved.