Add a content_shell flag to turn on partial site isolation.

content/shell/common/shell_content_client.cc

Index: content/shell/common/shell_content_client.cc
diff --git a/content/shell/common/shell_content_client.cc b/content/shell/common/shell_content_client.cc
index 2e420895e28bb983ca93e6fcce028d1650d491ef..b9b97126536df19ce971cb825c39c11a8b03b595 100644
--- a/content/shell/common/shell_content_client.cc
+++ b/content/shell/common/shell_content_client.cc
@@ -91,4 +91,9 @@ gfx::Image& ShellContentClient::GetNativeImageNamed(int resource_id) const {
   return ResourceBundle::GetSharedInstance().GetNativeImageNamed(resource_id);
 }
 
+bool ShellContentClient::IsSupplementarySiteIsolationModeEnabled() {
+  return base::CommandLine::ForCurrentProcess()->HasSwitch(
+      switches::kIsolateSitesForTesting);
+}
+
 }  // namespace content