Add isolate for content_browsertests with --site-per-process.

testing/buildbot/chromium.linux.json

 {
   "Android GN": {
     "additional_compile_targets": [
       "all"
     ],
     "gtest_tests": [
       {
         "test": "android_webview_unittests"
       },
       {
@@ skipping 392 matching lines @@
         },
         "test": "compositor_unittests"
       },
       {
         "swarming": {
           "can_use_on_swarming_builders": true
         },
         "test": "content_browsertests"
       },
       {
+        "override_compile_targets": [
+          "content_site_isolation_browsertests"
+        ],
+        "swarming": {
+          "can_use_on_swarming_builders": true
+        },
+        "test": "content_site_isolation_browsertests"
+      },
+      {
         "swarming": {
           "can_use_on_swarming_builders": true
         },
         "test": "content_unittests"
       },
       {
         "swarming": {
           "can_use_on_swarming_builders": true
         },
         "test": "crypto_unittests"
@@ skipping 1000 matching lines @@
       }
     ],
     "scripts": [
       {
         "name": "nacl_integration",
         "script": "nacl_integration.py"
       }
     ]
   }
 }