Pepper: Simplify FileIO for only one Open() call.

FileIO resource hosts should not support multiple calls to Open(). I'm not sure
why the code had a map for multiple pending open calls, but it seems
unnecessary, and makes the code more complicated than necessary.

This removes that lookup logic and cleans up a couple small style nits.

R=dmichael@chromium.org
TBR=jochen, jschuh
BUG=246396

Review URL: https://codereview.chromium.org/26552004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@227970 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 227970