PlzNavigate: implement process reuse for ServiceWorkers

content/browser/site_instance_impl.h

Index: content/browser/site_instance_impl.h
diff --git a/content/browser/site_instance_impl.h b/content/browser/site_instance_impl.h
index 4898b17c0d3d9c3d450e051195ad8f3e901233a4..6c2f058a4913349fd62ad4ce584cd3ab5c87be85 100644
--- a/content/browser/site_instance_impl.h
+++ b/content/browser/site_instance_impl.h
@@ -64,6 +64,13 @@ class CONTENT_EXPORT SiteInstanceImpl final : public SiteInstance,
     // In this mode, subframes will be hosted in a designated RenderProcessHost.
     USE_DEFAULT_SUBFRAME_PROCESS,
 
+    // In this mode, the site will be rendered in a RenderProcessHost that is
+    // already in use for the site, either for a pending navigation or a
+    // committed navigation.  If none exists, a new process will be created.  If
+    // multiple such processes exist, ones that have foreground frames are given
+    // priority, and otherwise one is selected randomly.
+    REUSE_PENDING_OR_COMMITTED_SITE,
+
     // By default, a new RenderProcessHost will be created unless the process
     // limit has been reached. The RenderProcessHost reused will be chosen
     // randomly and not based on the site.