Pepper: Drop some renderer dependencies in FileIO.

There are some unnecessary header includes and members in PepperFileIOHost that
make it dependent on being in the renderer. This takes care of the ones that
can be easily removed, making it simpler to move PepperFileIOHost to the
browser.

BUG=246396

Review URL: https://chromiumcodereview.appspot.com/23834012

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@224729 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 224729