Revert 238036 "Revert 238025 "Let test_isolation_mode default to..."

The change was reverted because swarming didn't work with branded builds,
but r238430 should've fixed that. Relanding. (There are no branded trybots.)

> Revert 238025 "Let test_isolation_mode default to check on mac."
> 
> Failing on "Google Chrome Mac" (http://build.chromium.org/p/chromium.chrome/builders/Google%20Chrome%20Mac/builds/15042)
> In the error log, tools/swarming_client/isolate.py says "File /Volumes/data/b/build/slave/google-chrome-rel-mac/build/src/out/Release/Chromium Framework.framework doesn't exist"
> 
> > 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
> > 
> > Review URL: https://codereview.chromium.org/92303002
> 
> TBR=thakis@chromium.org
> 
> Review URL: https://codereview.chromium.org/98383002

TBR=hashimoto@chromium.org

Review URL: https://codereview.chromium.org/102963003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@238466 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 238466