HTTPS Previews: Move up the early return in WillProcessResponse

I got a crash earlier today where WillProcessResponse was being called
on a non-preview page without response headers, so the DCHECK failed.

Moving up the early return makes the navigation throttle more robust
against this in the future.

Bug: 864635
Change-Id: Ief8e3b2b0ac9179d2ba16ff8f04c950b08f9564d
Reviewed-on: https://chromium-review.googlesource.com/c/1265059
Commit-Queue: Robert Ogden <robertogden@chromium.org>
Reviewed-by: Tarun Bansal <tbansal@chromium.org>
Cr-Commit-Position: refs/heads/master@{#597182}

chrome/browser/previews/previews_lite_page_navigation_throttle.cc

@@ -412,16 +412,15 @@ PreviewsLitePageNavigationThrottle::WillFailRequest() {
 
 content::NavigationThrottle::ThrottleCheckResult
 PreviewsLitePageNavigationThrottle::WillProcessResponse() {
-  const net::HttpResponseHeaders* response_headers =
-      navigation_handle()->GetResponseHeaders();
-  DCHECK(response_headers);
-
   std::string original_url;
   if (!GetOriginalURL(navigation_handle()->GetURL(), &original_url)) {
     // Return early if this request was not for a Preview.
     return content::NavigationThrottle::PROCEED;
   }
 
+  const net::HttpResponseHeaders* response_headers =
+      navigation_handle()->GetResponseHeaders();
+
   // After this point, the given response is known to be for a Preview.
   // The Previews server will only send the following response codes: 200, 307,
   // 404, and 503. 200 and 307 should proceed as normal, 404 and 503 request the