Add a Dns preresolve interface in //content

chrome/browser/chromeos/login/auth/auth_prewarmer.cc

Index: chrome/browser/chromeos/login/auth/auth_prewarmer.cc
diff --git a/chrome/browser/chromeos/login/auth/auth_prewarmer.cc b/chrome/browser/chromeos/login/auth/auth_prewarmer.cc
index 6ebf6bfa231768c524a921ac57b8ec7ab1bde069..881ec2ec096cbcc983da4b2fb6a5ce629987c0ff 100644
--- a/chrome/browser/chromeos/login/auth/auth_prewarmer.cc
+++ b/chrome/browser/chromeos/login/auth/auth_prewarmer.cc
@@ -97,8 +97,8 @@ void AuthPrewarmer::DoPrewarm() {
   content::BrowserThread::PostTask(
       content::BrowserThread::IO, FROM_HERE,
       base::Bind(&content::PreconnectUrl,
-                 base::RetainedRef(GetRequestContext()), url, url,
-                 kConnectionsNeeded, true,
+                 ProfileHelper::GetSigninProfile()->GetResourceContext(), url,

[eroman, 2016/05/31 20:45:27]

why this change? (i.e. as opposed to just removing base::RetainedRef())

(should be the same currently, but barring more information why not re-use the
existing abstraction of this class, GetRequestContext() ?)

[Charlie Harrison, 2016/05/31 21:14:47]

Talked to mmenke@ about it and he said ResourceContext is the better abstraction
than URLRequestContextGetter for this. Also, I have some plans to use the
ResourceContext to delegate back up to the ProfileIOData in //chrome RDHD, which
seems to be the common pattern there. 

+                 url, kConnectionsNeeded, true,
                  net::HttpRequestInfo::EARLY_LOAD_MOTIVATED));
   if (!completion_callback_.is_null()) {
     content::BrowserThread::PostTask(content::BrowserThread::UI, FROM_HERE,