[Swarming] Switch off test isolation check by default.

The steps are slow on dev workstations. Having them
run by the bots should be enough. The bots pass the mode
explicitly.

BUG=chromium:535160
LOG=n
TBR=tandrii@chromium.org, kjellander@chromium.org

Review URL: https://codereview.chromium.org/1738833002

Cr-Commit-Position: refs/heads/master@{#34290}

build/standalone.gypi

@@ -113,6 +113,8 @@
       # Check if valgrind directories are present.
       'has_valgrind%': '<!pymod_do_main(has_valgrind)',
 
+      'test_isolation_mode%': 'noop',
+
       'conditions': [
         # Set default gomadir.
         ['OS=="win"', {
@@ -136,14 +138,6 @@
         }, {
           'linux_use_bundled_gold%': 0,
         }],
-
-        # TODO(machenbach): Remove the conditions as more configurations are
-        # supported.
-        ['OS=="linux" or OS=="win"', {
-          'test_isolation_mode%': 'check',
-        }, {
-          'test_isolation_mode%': 'noop',
-        }],
       ],
     },
     'base_dir%': '<(base_dir)',