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

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

Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=238466

[Nico, 2013-12-03 22:38:28]

Committed patchset #1 manually as r238466.