Revert the merge of CrossSiteResourceHandler and NavigationResourceThrottle.

content/browser/devtools/render_frame_devtools_agent_host.cc

Index: content/browser/devtools/render_frame_devtools_agent_host.cc
diff --git a/content/browser/devtools/render_frame_devtools_agent_host.cc b/content/browser/devtools/render_frame_devtools_agent_host.cc
index f6c53011b8cb51f2b54b1e36c01029f7a033e3b6..a3985bf3c3ada34e6e74eb24c7d4701cfd82c00b 100644
--- a/content/browser/devtools/render_frame_devtools_agent_host.cc
+++ b/content/browser/devtools/render_frame_devtools_agent_host.cc
@@ -569,11 +569,8 @@ RenderFrameDevToolsAgentHost::~RenderFrameDevToolsAgentHost() {
 
 void RenderFrameDevToolsAgentHost::ReadyToCommitNavigation(
     NavigationHandle* navigation_handle) {
-  // TODO(clamy): Switch RenderFrameDevToolsAgentHost to always buffer messages
-  // until ReadyToCommitNavigation is called, now that it is also called in
-  // non-PlzNavigate mode.
-  if (!IsBrowserSideNavigationEnabled())
-    return;
+  // ReadyToCommitNavigation should only be called in PlzNavigate.
+  DCHECK(IsBrowserSideNavigationEnabled());
 
   // If the navigation is not tracked, return;
   if (navigating_handles_.count(navigation_handle) == 0)