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

content/test/BUILD.gn

Index: content/test/BUILD.gn
diff --git a/content/test/BUILD.gn b/content/test/BUILD.gn
index fc8a4151f7a8cf33a2991784480ee526ea84e320..5317f6c18d958329ffc33d464e46af771e9a9885 100644
--- a/content/test/BUILD.gn
+++ b/content/test/BUILD.gn
@@ -339,6 +339,25 @@ if (is_android) {
   }
 }
 
+# TODO(GYP): Delete this after we've converted everything to GN.
+# The _run targets exist only for compatibility w/ GYP.
+group("content_site_isolation_browsertests_run") {
+  testonly = true
+  deps = [
+    ":content_browsertests",
+  ]
+}
+
+group("content_site_isolation_browsertests") {
+  testonly = true
+  data = [
+    "//testing/buildbot/filters/site-per-process.content_browsertests.filter",
+  ]
+  data_deps = [
+    ":content_browsertests",
+  ]
+}
+
 test("content_browsertests") {
   sources = rebase_path(content_tests_gypi_values.content_browsertests_sources,
                         ".",