Use blocking worker pool for sync file operations in Pepper.

Use blocking worker pool for sync file operations in Pepper.

Switch from posting file operations on the file thread to posting them
on the blocking worker pool.  Previous instrumentation indicated that
this reduces hung-plugin infobars by around 20%.

BUG=153383
Committed: http://src.chromium.org/viewvc/chrome?view=rev&revision=161459

[Scott Hess - ex-Googler, 2012-10-10 22:47:50]

To land after revert in
  https://codereview.chromium.org/11087068/

After sending a try, I realized that without the other CL, it won't work.  I put
a combined version at:
   https://codereview.chromium.org/11092064
for purposes of running trybots.

[yzshen1, 2012-10-10 22:52:29]

lgtm

[Scott Hess - ex-Googler, 2012-10-10 23:32:14]

brettw for rubberstamp.  the earlier comment refs a cl which shows final result
of all this (revert CL then this one).

[brettw, 2012-10-11 21:23:16]

lgtm

[commit-bot: I haz the power, 2012-10-11 21:27:24]

CQ is trying da patch. Follow status at
https://chromium-status.appspot.com/cq/shess@chromium.org/11093059/1

[commit-bot: I haz the power, 2012-10-12 00:33:56]

Change committed as 161459