Add feature to control lower JS priority when DeferAllScript enabled

Add feature to not lower JavaScript priority when
DeferAllScript intervention is enabled.

Change-Id: I6a2fdc364398c39ae171a1e23b61e2819d4e68b6
Bug: 978490
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1779167Reviewed-by: Doug Arnett <dougarnett@chromium.org>
Reviewed-by: Kinuko Yasuda <kinuko@chromium.org>
Commit-Queue: Tarun Bansal <tbansal@chromium.org>
Cr-Commit-Position: refs/heads/master@{#693199}

third_party/blink/common/features.cc

@@ -429,5 +429,11 @@ const base::Feature kIgnoreCrossOriginWindowWhenNamedAccessOnWindow{
     "IgnoreCrossOriginWindowWhenNamedAccessOnWindow",
     base::FEATURE_DISABLED_BY_DEFAULT};
 
+// When enabled, loading priority of JavaScript requests is lowered when they
+// are force deferred by the intervention.
+const base::Feature kLowerJavaScriptPriorityWhenForceDeferred{
+    "LowerJavaScriptPriorityWhenForceDeferred",
+    base::FEATURE_ENABLED_BY_DEFAULT};
+
 }  // namespace features
 }  // namespace blink

third_party/blink/public/common/features.h

@@ -134,6 +134,9 @@ BLINK_COMMON_EXPORT extern const base::Feature
 BLINK_COMMON_EXPORT extern const base::Feature
     kIgnoreCrossOriginWindowWhenNamedAccessOnWindow;
 
+BLINK_COMMON_EXPORT extern const base::Feature
+    kLowerJavaScriptPriorityWhenForceDeferred;
+
 }  // namespace features
 }  // namespace blink
 

third_party/blink/renderer/platform/loader/fetch/resource_fetcher.cc

@@ -32,8 +32,10 @@
 #include <utility>
 
 #include "base/auto_reset.h"
+#include "base/feature_list.h"
 #include "base/time/time.h"
 #include "services/network/public/cpp/request_mode.h"
+#include "third_party/blink/public/common/features.h"
 #include "third_party/blink/public/common/loader/request_destination.h"
 #include "third_party/blink/public/common/mime_util/mime_util.h"
 #include "third_party/blink/public/mojom/devtools/console_message.mojom-blink.h"
@@ -254,6 +256,11 @@ ResourceLoadPriority AdjustPriorityWithDeferScriptIntervention(
     const ResourceRequest& resource_request,
     FetchParameters::DeferOption defer_option,
     bool is_link_preload) {
+  if (!base::FeatureList::IsEnabled(
+          blink::features::kLowerJavaScriptPriorityWhenForceDeferred)) {
+    return priority_so_far;
+  }
+
   WebURLRequest::PreviewsState context_previews_state =
       fetch_context.previews_state();