Modify SecurityOrigin before SetSecurityOrigin() in WorkerGlobalScope

To reduce modifications to SecurityContext's SecurityOrigin and
make it easier to make SecurityContext's SecurityOrigin const.
No behavior changes are expected.

Bug: 779730
Change-Id: I03ac5b2a22eb84cce8cd287eed3a4c13cb639f2f
Reviewed-on: https://chromium-review.googlesource.com/762360
Commit-Queue: Hiroshige Hayashizaki <hiroshige@chromium.org>
Reviewed-by: Hiroki Nakagawa <nhiroki@chromium.org>
Cr-Commit-Position: refs/heads/master@{#519743}

third_party/WebKit/Source/core/workers/WorkerGlobalScope.cpp

@@ -327,11 +327,12 @@ WorkerGlobalScope::WorkerGlobalScope(
       font_selector_(OffscreenFontSelector::Create(this)) {
   InstanceCounters::IncrementCounter(
       InstanceCounters::kWorkerGlobalScopeCounter);
-  SetSecurityOrigin(SecurityOrigin::Create(url_));
+  scoped_refptr<SecurityOrigin> security_origin = SecurityOrigin::Create(url_);
   if (creation_params->starter_origin) {
-    GetSecurityOrigin()->TransferPrivilegesFrom(
+    security_origin->TransferPrivilegesFrom(
         creation_params->starter_origin->CreatePrivilegeData());
   }
+  SetSecurityOrigin(std::move(security_origin));
   ApplyContentSecurityPolicyFromVector(
       *creation_params->content_security_policy_parsed_headers);
   SetWorkerSettings(std::move(creation_params->worker_settings));