Revert 251054 "Make all the files mapped in when running unit_te..."

Speculative revert, to see if this fixes CrOS MergeSession tests.

> Make all the files mapped in when running unit_tests read only on Linux and OSX.
> 
> This means no file can be opened for write during the test run.
> 
> Windows can't be done at the moment due to bug 342913.
> 
> R=vadimsh@chromium.org
> BUG=116251
> BUG=342913
> 
> Review URL: https://codereview.chromium.org/136973003

TBR=maruel@chromium.org

Review URL: https://codereview.chromium.org/166483006

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@251411 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 251411