Only use SiteInstance process reuse for ServiceWorker with no failures

content/browser/service_worker/service_worker_process_manager.cc

Index: content/browser/service_worker/service_worker_process_manager.cc
diff --git a/content/browser/service_worker/service_worker_process_manager.cc b/content/browser/service_worker/service_worker_process_manager.cc
index b7fbe590ba6c1bc747066ef2c9f6c6d26fe3bfdd..cedb5d9c5d14a2caac94e0713bed7ae7b58685a8 100644
--- a/content/browser/service_worker/service_worker_process_manager.cc
+++ b/content/browser/service_worker/service_worker_process_manager.cc
@@ -216,14 +216,18 @@ void ServiceWorkerProcessManager::AllocateWorkerProcess(
     }
   }
 
-  // No existing processes available; start a new one.
+  // ServiceWorkerProcessManager does not know of any renderer processes that
+  // are available for |pattern|. Create a SiteInstance and ask for a renderer
+  // process. Attempt to reuse an existing process if possible.
   // TODO(clamy): Update the process reuse mechanism above following the
   // implementation of
   // SiteInstanceImpl::ProcessReusePolicy::REUSE_PENDING_OR_COMMITTED_SITE.
   scoped_refptr<SiteInstanceImpl> site_instance =
       SiteInstanceImpl::CreateForURL(browser_context_, script_url);

[falken, 2017/06/07 00:12:22]

does it make sense to DCHECK_NE(REUSE_PENDING_OR_COMMITTED_SITE)? Or can we
DCHECK something more specific?

I'm just wondering if we're sure that CreateForURL will create a new
SiteInstanceImpl that definitely doesn't have the REUSE flag set (i.e., it's not
going to return an existing SiteInstanceImpl). Reading the code comments and
impl, that indeed seems true, so we could DCHECK this assumption?

-  site_instance->set_process_reuse_policy(
-      SiteInstanceImpl::ProcessReusePolicy::REUSE_PENDING_OR_COMMITTED_SITE);
+  if (can_use_existing_process) {
+    site_instance->set_process_reuse_policy(
+        SiteInstanceImpl::ProcessReusePolicy::REUSE_PENDING_OR_COMMITTED_SITE);
+  }
   RenderProcessHost* rph = site_instance->GetProcess();
 
   // This Init() call posts a task to the IO thread that adds the RPH's