DCHECK(IsMainFrame) when trying to reuse process on cross-BI history nav

This was previously used as a condition instead of a DCHECK because of
crbug.com/1107269.

Bug: 1107269
Change-Id: I16c814fb60b40bb508c8fa43fa91f41b27feb5af
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2359970
Auto-Submit: Rakina Zata Amni <rakina@chromium.org>
Commit-Queue: Charlie Reis <creis@chromium.org>
Reviewed-by: Charlie Reis <creis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#799260}

content/browser/frame_host/render_frame_host_manager.cc

@@ -1577,13 +1577,11 @@ RenderFrameHostManager::GetSiteInstanceForNavigation(
       IsProactivelySwapBrowsingInstanceOnSameSiteNavigationEnabled() ||
       IsSameSiteBackForwardCacheEnabled();
   if (is_same_site_proactive_swap_enabled && is_history_navigation &&
-      swapped_browsing_instance && frame_tree_node_->IsMainFrame() &&
+      swapped_browsing_instance &&
       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.
+    DCHECK(frame_tree_node_->IsMainFrame());
     new_instance_impl->ReuseCurrentProcessIfPossible(
         current_instance->GetProcess());
   }