Do PPB_FileIO Query and Read in the plugin process.
This changes Open to send a file descriptor to the plugin process on Open.
The file descriptor is used on the plugin side to implement Query and Read.
The implementation reuses base::FileUtilProxy to use a file thread on the
plugin side to call potentially blocking file functions.
TBR=dmichael@chromium.org, jschuh@chromium.org
BUG=194304
TEST=browser_tests, PPAPITest.FileIO
Review URL: https://codereview.chromium.org/
21016004
git-svn-id: svn://svn.chromium.org/chrome/trunk/src@214420 0039d316-1c4b-4281-b951-d872f2087c98