gn BUILD fixes for disabling enable_extensions and use_ash feature flags.

testing/buildbot/chromium.linux.json

 {
   "Android Arm64 Builder (dbg)": {
     "additional_compile_targets": [
       "gn_all"
     ]
   },
   "Android Builder": {
     "additional_compile_targets": [
       "cronet_test_instrumentation_apk",
       "system_webview_apk"
@@ skipping 766 matching lines @@
       {
         "swarming": {
           "can_use_on_swarming_builders": true
         },
         "test": "leveldb_service_unittests"
       },
       {
         "swarming": {
           "can_use_on_swarming_builders": true
         },
-        "test": "mash_unittests"
-      },
-      {
-        "swarming": {
-          "can_use_on_swarming_builders": true
-        },
         "test": "media_blink_unittests"
       },
       {
         "swarming": {
           "can_use_on_swarming_builders": true
         },
         "test": "media_mojo_shell_unittests"
       },
       {
         "swarming": {
@@ skipping 1007 matching lines @@
         "override_compile_targets": [
           "telemetry_unittests_run"
         ],
         "swarming": {
           "can_use_on_swarming_builders": true
         }
       }
     ]
   }
 }