Only process frame-ancestors CSP for response checks.

Bug: 759184
Change-Id: I3cb3b30d7c52c6ce174833702057d9c323f688f1
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1942870Reviewed-by: Arthur Sonzogni <arthursonzogni@chromium.org>
Commit-Queue: Lucas Gadani <lfg@chromium.org>
Cr-Commit-Position: refs/heads/master@{#720005}

content/browser/frame_host/ancestor_throttle.cc

@@ -183,8 +183,8 @@ NavigationThrottle::ThrottleCheckResult AncestorThrottle::ProcessResponseImpl(
 
   // Evaluate whether the navigation should be allowed or blocked based on
   // existing content-security-policy on the response.
-  if (base::FeatureList::IsEnabled(
-          network::features::kOutOfBlinkFrameAncestors)) {
+  if (is_response_check && base::FeatureList::IsEnabled(
+                               network::features::kOutOfBlinkFrameAncestors)) {
     if (network::mojom::ContentSecurityPolicyPtr policy =
             request->response()->head.content_security_policy) {
       // TODO(arthursonzogni): Remove content::ContentSecurityPolicy in favor of