Revert 251411 "Revert 251054 "Make all the files mapped in when ..."

Apparently, couldn't be the cause. Reverting my revert.

> 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

TBR=asvitkine@chromium.org

Review URL: https://codereview.chromium.org/164183010

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@251426 0039d316-1c4b-4281-b951-d872f2087c98

chrome/unit_tests.isolate

@@ -73,6 +73,12 @@
         ],
       },
     }],
+    ['OS=="linux" or OS=="mac"', {
+      'variables': {
+        # Can't be enabled on Windows due to http://crbug.com/342913
+        'read_only': 1,
+      },
+    }],
     ['OS=="mac"', {
       'variables': {
         'isolate_dependency_tracked': [