Use blocking worker pool for sync file operations in Pepper.

[Manually closed, I think something similar was already landed.]

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
Revert: Flapper field trial to use workerpool for sync file operations.

Original: https://chromiumcodereview.appspot.com/11016016

The field trial indicated a 20% improvement using blocking workerpool.
That seems worth merging to M-23.  Revert the change so that a clean
change can be landed and drover'ed over.

BUG=153383