gn BUILD fixes for disabling enable_extensions and use_ash feature flags.

testing/buildbot/chromium.win.json

 {
   "Win 7 Tests x64 (1)": {
     "gtest_tests": [
       {
         "swarming": {
           "can_use_on_swarming_builders": true
         },
         "test": "accessibility_unittests"
       },
       {
@@ skipping 2138 matching lines @@
       "gcm_unit_tests",
       "gfx_unittests",
       "google_apis_unittests",
       "gpu_ipc_service_unittests",
       "gpu_unittests",
       "interactive_ui_tests",
       "ipc_mojo_unittests",
       "ipc_tests",
       "jingle_unittests",
       "keyboard_unittests",
-      "mash:all",
       "media_unittests",
       "media_blink_unittests",
       "message_center_unittests",
       "ppapi_unittests",
       "printing_unittests",
       "remoting_all",
       "sbox_integration_tests",
       "sbox_unittests",
       "sbox_validation_tests",
       "skia_unittests",
@@ skipping 78 matching lines @@
         },
         "test": "mojo_system_unittests"
       },
       {
         "swarming": {
           "can_use_on_swarming_builders": true
         },
         "test": "mus_public_unittests"
       },
       {
-        "args": [
-          "--override-use-gl-with-osmesa-for-tests"
-        ],
-        "swarming": {
-          "can_use_on_swarming_builders": true
-        },
-        "test": "mash_unittests"
-      },
-      {
         "test": "mus_gpu_unittests"
       },
       {
         "test": "mus_ws_unittests"
       },
       {
         "swarming": {
           "can_use_on_swarming_builders": true
         },
         "test": "navigation_unittests"
@@ skipping 16 matching lines @@
       },
       {
         "swarming": {
           "can_use_on_swarming_builders": true
         },
         "test": "events_unittests"
       }
     ]
   }
 }