Convert extensions_browsertests to run exclusively on Swarming

testing/buildbot/chromium.memory.json

 {
   "Linux ASan LSan Tests (1)": {
     "gtest_tests": [
       {
         "swarming": {
           "can_use_on_swarming_builders": true
         },
         "test": "accessibility_unittests"
       },
       {
@@ skipping 48 matching lines @@
         },
         "test": "crypto_unittests"
       },
       {
         "test": "device_unittests"
       },
       {
         "test": "display_unittests"
       },
       {
+        "swarming": {
+          "can_use_on_swarming_builders": true
+        },
         "test": "extensions_browsertests"
       },
       {
         "test": "extensions_unittests"
       },
       {
         "test": "gcm_unit_tests"
       },
       {
         "test": "gfx_unittests"
@@ skipping 386 matching lines @@
           "can_use_on_swarming_builders": true
         },
         "test": "unit_tests"
       },
       {
         "test": "url_unittests"
       }
     ]
   }
 }