Reuse process on history navigations between same-site cross-BI pages

See crbug.com/1096135 for context. When doing same-site navigations, we
might do a proactive BrowsingInstance swap, but we will reuse the
renderer process whenever we can. On history navigations between pages
that are same-site but have different BrowsingInstances (not only on
cases that are caused by proactive BI swap), we should try to reuse the
renderer process.

Bug: 1096135
Change-Id: I58f72bedecbbf94e4e05218bb9c15970a502d8a9
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2282553
Commit-Queue: Rakina Zata Amni <rakina@chromium.org>
Reviewed-by: Charlie Reis <creis@chromium.org>
Reviewed-by: Alex Moshchuk <alexmos@chromium.org>
Cr-Commit-Position: refs/heads/master@{#792195}

content/browser/frame_host/render_frame_host_manager.cc

@@ -1538,6 +1538,26 @@ RenderFrameHostManager::GetSiteInstanceForNavigation(
     new_instance_impl->ReuseCurrentProcessIfPossible(
         current_instance->GetProcess());
   }
+
+  // If this is a same-site history navigation with different BrowsingInstances,
+  // the original navigation might have done a proactive BrowsingInstance swap,
+  // which means we should try to reuse the current process (because we did too
+  // on the original navigation).
+  bool is_history_navigation = !!dest_instance;
+  bool swapped_browsing_instance =
+      !new_instance->IsRelatedSiteInstance(current_instance);
+  if (IsProactivelySwapBrowsingInstanceOnSameSiteNavigationEnabled() &&
+      is_history_navigation && swapped_browsing_instance &&
+      frame_tree_node_->IsMainFrame() &&
+      IsCurrentlySameSite(
+          static_cast<RenderFrameHostImpl*>(render_frame_host_.get()),
+          dest_url)) {
+    // TODO(crbug.com/1107269): DCHECK for frame_tree_node_->IsMainFrame() once
+    // we can guarantee all cross-BrowsingInstance history navigations only
+    // happen on main frames.
+    new_instance_impl->ReuseCurrentProcessIfPossible(
+        current_instance->GetProcess());
+  }
   return new_instance;
 }
 

content/browser/site_instance_impl.cc

@@ -359,6 +359,9 @@ void SiteInstanceImpl::ReuseCurrentProcessIfPossible(
           current_process, GetIsolationContext(), site_info_, IsGuest())) {
     return;
   }
+
+  // TODO(crbug.com/1055779 ): Don't try to reuse process if either of the
+  // SiteInstances are cross-origin isolated (uses COOP/COEP).
   SetProcessInternal(current_process);
 }