Revert of Let test_isolation_mode default to check on mac.

build/common.gypi

Index: build/common.gypi
diff --git a/build/common.gypi b/build/common.gypi
index 55d00e1b67c2cdb94ae457f62877137b81106384..97e8596bd021e114af6db6b25e4439b7ddb5c89b 100644
--- a/build/common.gypi
+++ b/build/common.gypi
@@ -756,10 +756,12 @@
         # 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!="ios" and OS!="android" and chromeos==0', {
+        ['OS!="mac" and OS!="ios" and OS!="android" and chromeos==0', {
           'test_isolation_mode%': 'check',
         }, {
           'test_isolation_mode%': 'noop',