Add KeepScriptResourceAlive feature and flag

This CL puts the KeepScriptResourceAlive base::Feature under the
blink::features namespace. It also adds a corresponding about:flags
flag, and RuntimeEnabledFeature.

ScriptLoader's behavior depends on the RuntimeEnabledFeature, which is
enabled when the base::Feature is enabled. The base::Feature is enabled
when the corresponding about:flags flag is enabled.

These are the first steps to running experiments with the
KeepScriptResourceAlive feature. A subsequent CL will add to
fieldtrial_testing_conflig.json, so we can control the base::Feature via
Finch.

R=kinuko@chromium.org

Bug: 1043679
Change-Id: Ib7cd83346c6634dad547311100a614cc43e0fce6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2112172Reviewed-by: Kinuko Yasuda <kinuko@chromium.org>
Commit-Queue: Dominic Farolino <dom@chromium.org>
Cr-Commit-Position: refs/heads/master@{#752772}

third_party/blink/common/features.cc

@@ -496,5 +496,8 @@ const base::FeatureParam<int> kFontPreloadingDelaysRenderingParam{
 
 const base::Feature kFlexNG{"FlexNG", base::FEATURE_DISABLED_BY_DEFAULT};
 
+const base::Feature kKeepScriptResourceAlive{"KeepScriptResourceAlive",
+                                             base::FEATURE_DISABLED_BY_DEFAULT};
+
 }  // namespace features
 }  // namespace blink

third_party/blink/public/common/features.h

@@ -164,6 +164,8 @@ BLINK_COMMON_EXPORT extern const base::FeatureParam<int>
 
 BLINK_COMMON_EXPORT extern const base::Feature kFlexNG;
 
+BLINK_COMMON_EXPORT extern const base::Feature kKeepScriptResourceAlive;
+
 }  // namespace features
 }  // namespace blink
 

third_party/blink/renderer/core/script/script_loader.cc

@@ -26,6 +26,7 @@
 
 #include "base/feature_list.h"
 #include "third_party/blink/public/common/feature_policy/feature_policy.h"
+#include "third_party/blink/public/common/features.h"
 #include "third_party/blink/public/mojom/feature_policy/feature_policy_feature.mojom-blink.h"
 #include "third_party/blink/renderer/bindings/core/v8/sanitize_script_errors.h"
 #include "third_party/blink/renderer/bindings/core/v8/v8_binding_for_core.h"
@@ -72,13 +73,6 @@
 
 namespace blink {
 
-namespace {
-
-const base::Feature kKeepScriptResourceAlive{"KeepScriptResourceAlive",
-                                             base::FEATURE_DISABLED_BY_DEFAULT};
-
-}
-
 ScriptLoader::ScriptLoader(ScriptElementBase* element,
                            bool parser_inserted,
                            bool already_started)
@@ -982,7 +976,8 @@ void ScriptLoader::PendingScriptFinished(PendingScript* pending_script) {
   // keep the resource in the memory cache.
   if (resource_keep_alive_ &&
       !resource_keep_alive_->GetResponse().IsSignedExchangeInnerResponse() &&
-      !base::FeatureList::IsEnabled(kKeepScriptResourceAlive)) {
+      !base::FeatureList::IsEnabled(
+          blink::features::kKeepScriptResourceAlive)) {
     resource_keep_alive_ = nullptr;
   }