Cache User Agent string in BlinkPlatformImpl

content/child/blink_platform_impl.cc

Index: content/child/blink_platform_impl.cc
diff --git a/content/child/blink_platform_impl.cc b/content/child/blink_platform_impl.cc
index 1a1cc6781df243748829e2392f188715b1ae2923..135cc72342717fd22fb58c3a7dab3538991a069f 100644
--- a/content/child/blink_platform_impl.cc
+++ b/content/child/blink_platform_impl.cc
@@ -487,7 +487,12 @@ blink::WebSocketHandle* BlinkPlatformImpl::createWebSocketHandle() {
 }
 
 WebString BlinkPlatformImpl::userAgent() {
-  return WebString::fromUTF8(GetContentClient()->GetUserAgent());
+  CR_DEFINE_STATIC_LOCAL(
+      blink::WebString, user_agent,
+      (blink::WebString::fromUTF8(GetContentClient()->GetUserAgent())));
+  DCHECK(user_agent ==
+         blink::WebString::fromUTF8(GetContentClient()->GetUserAgent()));
+  return user_agent;
 }
 
 WebData BlinkPlatformImpl::parseDataURL(const WebURL& url,