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: http://src.chromium.org/svn/trunk/src/build@238466 4ff67af0-8c30-449e-8e8b-ad334ec8d88c

common.gypi

@@ -748,12 +748,10 @@
         # overriden with GYP_DEFINES.
         # TODO(maruel): Remove the conditions as more configurations are
         # supported.
-        # TODO(csharp): Remove OS!="mac" once xcode can run the isolate code
-        # again.
         # NOTE: The check for disable_nacl==0 and component=="static_library"
         # can't be used here because these variables are not defined yet, but it
         # is still not supported.
-        ['OS!="mac" and OS!="ios" and OS!="android" and chromeos==0', {
+        ['OS!="ios" and OS!="android" and chromeos==0', {
           'test_isolation_mode%': 'check',
         }, {
           'test_isolation_mode%': 'noop',