Make all the files mapped in when running browser_tests read only.

This means no file can be opened for write during the test run.

R=vadimsh@chromium.org
NOTRY=true
BUG=116251

Review URL: https://codereview.chromium.org/137133002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@251074 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 251074