Bug 854319 - Part 1: Call BeginDecoding before OnDecodeThreadStart; r=cpearce a=akeybl

This is what WMFReader::OnDecodeThreadStart() expects.

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extra : transplant_source : z%0E%B8%B3%04%EBi%86%C1%CC%DB%CFM%E4g%A2%A5%E6%9E%94