Pepper browser tests: Only grant private permissions to private tests.

This allows us to exercise the quota checking machinery for PPB_FileIO.

BUG=194304

Review URL: https://codereview.chromium.org/115693008

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@242545 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 242545