Add dynamic test configurations for Site Isolation FYI Builders.

testing/buildbot/chromium.fyi.json

Index: testing/buildbot/chromium.fyi.json
diff --git a/testing/buildbot/chromium.fyi.json b/testing/buildbot/chromium.fyi.json
index 1c97562dd3477936c76cb50de7c858e4c609b943..d25ebd53019dbf6480b4c4bcb3a3edbd68706990 100644
--- a/testing/buildbot/chromium.fyi.json
+++ b/testing/buildbot/chromium.fyi.json
@@ -273,5 +273,17 @@
       "url_unittests",
       "browser_tests"
     ]
+  },
+  "Site Isolation Linux": {
+    "gtest_tests": [
+      { "test": "content_unittests", "args": ["--site-per-process"] },
+      { "test": "content_browsertests", "args": ["--site-per-process"] }
+    ]
+  },
+  "Site Isolation Win": {
+    "gtest_tests": [
+      { "test": "content_unittests", "args": ["--site-per-process"] },
+      { "test": "content_browsertests", "args": ["--site-per-process"] }
+    ]
   }
 }