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 f303dca0ff5ee66b962191f92469cef78ac7bb6d..18bfe60becc090fa540cdb7238980eabf19ff277 100644
--- a/content/test/BUILD.gn
+++ b/content/test/BUILD.gn
@@ -313,6 +313,22 @@ group("content_browsertests_run") {
   ]
 }
 
+# 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_deps = [
+    ":content_browsertests",
+  ]
+}
+
 test("content_browsertests") {
   sources = rebase_path(content_tests_gypi_values.content_browsertests_sources,
                         ".",