Restore an accidentally deleted comment in DetermineSiteInstanceForURL

Looks like one line of a comment near the end of this function was 
accidentally deleted in https://chromium-review.googlesource.com/c/chromium/src/+/2230632.  This CL restores it.

Change-Id: I081e0a184f2468e5e6e60a63446a9fa5cca2a0f3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2626907
Auto-Submit: Alex Moshchuk <alexmos@chromium.org>
Reviewed-by: Arthur Hemery <ahemery@chromium.org>
Commit-Queue: Alex Moshchuk <alexmos@chromium.org>
Cr-Commit-Position: refs/heads/master@{#842908}

content/browser/renderer_host/render_frame_host_manager.cc

@@ -2086,7 +2086,8 @@ RenderFrameHostManager::DetermineSiteInstanceForURL(
     }
   }
 
-  // BrowsingInstance unless the destination URL's cross-origin isolated state
+  // Start the new renderer in a new SiteInstance, but in the current
+  // BrowsingInstance, unless the destination URL's cross-origin isolated state
   // cannot be hosted by it.
   if (IsSiteInstanceCompatibleWithCoopCoepCrossOriginIsolation(
           render_frame_host_->GetSiteInstance(),