Clean up no-op core/fetch Priority Hints leftovers

This CL cleans up some unused code that would otherwise be used if we
were exposing `importance` as a readonly attribute on the Request
interface, but we are not right now. This also replaces a
RuntimeEnabledFeatures check for Priority Hints with a DCHECK since we
already check the flag earlier than the location this CL touches.

R=kinuko@chromium.org, kouhei@chromium.org, yhirano@chromium.org, yoav@yoav.ws

Bug: 821464
Change-Id: Ic1d2fe3ddabf4eeec46b5298cc4a914001cd2d32
Reviewed-on: https://chromium-review.googlesource.com/1092243Reviewed-by: Yoav Weiss <yoav@yoav.ws>
Reviewed-by: Kouhei Ueno <kouhei@chromium.org>
Reviewed-by: Yutaka Hirano <yhirano@chromium.org>
Reviewed-by: Kinuko Yasuda <kinuko@chromium.org>
Commit-Queue: Dominic Farolino <domfarolino@gmail.com>
Cr-Commit-Position: refs/heads/master@{#565926}

third_party/blink/renderer/core/fetch/request.cc

@@ -265,16 +265,16 @@ Request* Request::CreateRequestWithRequestOrString(
       request->SetMode(network::mojom::FetchRequestMode::kCORS);
   }
 
-  if (RuntimeEnabledFeatures::PriorityHintsEnabled()) {
-    // This is not yet standardized, but we can assume the following:
-    // "If |init|'s importance member is present, set |request|'s importance
-    // mode to it." For more information see Priority Hints at
-    // https://crbug.com/821464
-    if (init.Importance() == "low") {
-      request->SetImportance(mojom::FetchImportanceMode::kImportanceLow);
-    } else if (init.Importance() == "high") {
-      request->SetImportance(mojom::FetchImportanceMode::kImportanceHigh);
-    }
+  // This is not yet standardized, but we can assume the following:
+  // "If |init|'s importance member is present, set |request|'s importance
+  // mode to it." For more information see Priority Hints at
+  // https://crbug.com/821464
+  DCHECK(init.Importance().IsNull() ||
+         RuntimeEnabledFeatures::PriorityHintsEnabled());
+  if (init.Importance() == "low") {
+    request->SetImportance(mojom::FetchImportanceMode::kImportanceLow);
+  } else if (init.Importance() == "high") {
+    request->SetImportance(mojom::FetchImportanceMode::kImportanceHigh);
   }
 
   // "Let |credentials| be |init|'s credentials member if it is present, and

third_party/blink/renderer/core/fetch/request.h

@@ -69,7 +69,6 @@ class CORE_EXPORT Request final : public Body {
   String cache() const;
   String redirect() const;
   String integrity() const;
-  String importance() const;
   bool keepalive() const;
   bool isHistoryNavigation() const;
   AbortSignal* signal() const { return signal_; }

third_party/blink/renderer/core/fetch/request.idl

@@ -13,7 +13,6 @@ enum RequestCredentials { "omit", "same-origin", "include" };
 enum RequestRedirect { "follow", "error", "manual" };
 enum RequestCache { "default", "no-store", "reload", "no-cache", "force-cache", 
     "only-if-cached" };
-enum RequestImportance { "low", "auto", "high" };
 
 // https://w3c.github.io/webappsec-referrer-policy/#referrer-policies