File permission fix: now we selectively grant read permission for Sandboxed files

We also need to check the read permission and call GrantReadFile() for
sandboxed files for CreateSnapshotFile().

BUG=162114
TEST=manual

Review URL: https://codereview.chromium.org/11280231

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@170181 0039d316-1c4b-4281-b951-d872f2087c98