Revert 241705 "Revert 241681 "Let test_isolation_mode default to..."

This relands 241681, which should now hopefully work after r241717.

> Revert 241681 "Let test_isolation_mode default to check on mac."
> 
> Still not working on the branded builders, lots of .isolate files still
> hardcode "chromium framework" (see http://crbug.com/324911)
> 
> > Let test_isolation_mode default to check on mac.
> > 
> > It was set to noop in https://chromiumcodereview.appspot.com/14055005 due to
> > issues with xcode. Now that xcode is no longer used, it can be set to check
> > again.
> > 
> > BUG=none
> > 
> > Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=238025
> > 
> > R=maruel@chromium.org
> > 
> > Review URL: https://codereview.chromium.org/92303002
> 
> TBR=thakis@chromium.org
> 
> Review URL: https://codereview.chromium.org/110423005

TBR=thakis@chromium.org

Review URL: https://codereview.chromium.org/108293003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@241767 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 241767