Add a feature flag for network service and about:flags entry for it.

testing/buildbot/chromium.fyi.json

 {
   "Android Asan Builder Tests (dbg)": {
     "gtest_tests": [
       {
         "args": [
           "--tool=asan"
         ],
         "swarming": {
           "can_use_on_swarming_builders": false
         },
@@ skipping 10907 matching lines @@
           "can_use_on_swarming_builders": true,
           "shards": 4
         }
       }
     ]
   },
   "Mojo Linux": {
     "gtest_tests": [
       {
         "args": [
-          "--enable-network-service",
+          "--enable-features=NetworkService",
           "--test-launcher-filter-file=../../testing/buildbot/filters/mojo.fyi.network_content_browsertests.filter"
         ],
         "name": "network_service_content_browsertests",
         "swarming": {
           "can_use_on_swarming_builders": true
         },
         "test": "content_browsertests"
       }
     ],
     "isolated_scripts": [
       {
         "args": [
-          "--additional-driver-flag=--enable-network-service"
+          "--additional-driver-flag=--enable-features=NetworkService"
         ],
         "isolate_name": "webkit_layout_tests_exparchive",
         "merge": {
           "args": [
             "--verbose",
             "--results-json-override-with-build-property",
             "build_number",
             "buildnumber",
             "--results-json-override-with-build-property",
             "builder_name",
@@ skipping 1562 matching lines @@
       },
       {
         "swarming": {
           "can_use_on_swarming_builders": false
         },
         "test": "zucchini_unittests"
       }
     ]
   }
 }