Adding support for FileRef and FileSystem to postMessageAndAwaitResponse

[David Michael]

I have it working, and I'm in the process of splitting it in to easier-to-review parts. I wrote up a quick doc explaining the approach so it would help give context for the code reviews. I'm going to ask raymes@ to review, since I think he's most familiar with this area, but I wanted to send this to everyone in case you wanted to read it:

https://docs.google.com/a/chromium.org/document/d/1-BSlFJi_PQIet-av5FeMlEC5_BCZyapsonUPnx--6iY/edit#

Here's the next CL in line:
https://codereview.chromium.org/730603002/