More filesystem cleanup: convert URL-encoded-as-FilePath to actual URL, where

More filesystem cleanup: convert URL-encoded-as-FilePath to actual URL, where 
possible without WebKit API changes. The WebKit changes will happen in another 
CL. 

This is a resubmit of http://codereview.chromium.org/6767010/, which bounced
recently. There are a few changes here that weren't there [most notably in
file_system_operation_write_unittest.cc and file_system_operation.cc], but
they're pretty trivial build/test fixes.
Committed: http://src.chromium.org/viewvc/chrome?view=rev&revision=81463