Prerender: Explicitly avoid activation for prerendering navigation

Before this CL, activation for prerendering navigation is avoided by the
order of prerender start and map insertion in
PrerenderHostRegistry::StartPrerendering().

After this CL, it's explicitly avoided by checking
RenderFrameHostImpl::IsPrerendering() in
PrerenderHostRegistry::SelectForNavigation().

Bug: 1132746
Change-Id: I112cab26f6abd004554ef0033613ee5e919c5552
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2626683Reviewed-by: Matt Falkenhagen <falken@chromium.org>
Commit-Queue: Hiroki Nakagawa <nhiroki@chromium.org>
Cr-Commit-Position: refs/heads/master@{#843044}

content/browser/prerender/prerender_host_registry.cc

@@ -33,16 +33,7 @@ void PrerenderHostRegistry::CreateAndStartHost(
 
   auto prerender_host = std::make_unique<PrerenderHost>(
       std::move(attributes), initiator_render_frame_host_id, initiator_origin);
-
-  // Start prerendering before adding the host to the map to make sure
-  // navigation for prerendering doesn't select itself.
-  // TODO(https://crbug.com/1132746): SelectForNavigation() should avoid select
-  // a prerender host when the current NavigationRequest is for prerendering
-  // regardless of the calling order of StartPrerendering(). This modification
-  // would require the proper `is_prerendering` state in NavigationRequest,
-  // RenderFrameHostImpl, or somewhere else.
   prerender_host->StartPrerendering();
-
   prerender_host_by_url_[prerendering_url] = std::move(prerender_host);
 }
 
@@ -55,6 +46,10 @@ std::unique_ptr<PrerenderHost> PrerenderHostRegistry::SelectForNavigation(
     FrameTreeNode& frame_tree_node) {
   RenderFrameHostImpl* render_frame_host = frame_tree_node.current_frame_host();
 
+  // Disallow activation when the navigation is for prerendering.
+  if (render_frame_host->IsPrerendering())
+    return nullptr;
+
   // Disallow activation when the render frame host is for a nested browsing
   // context (e.g., iframes). This is because nested browsing contexts are
   // supposed to be created in the parent's browsing context group and can