Add a flag for experimental-productivity-features.

This flag enables a set of features which are intended to help improve
web developer productivity, by restricting the use of web-platform
features which are commonly problematic, as well as adding new high-
level APIs for common development patterns.

This currently just enables three opt-in policy-controlled features:
 - sync-script
 - unsized-media
 - vertical-scroll

Other policy-controlled features and LAPIs are expected to be added to
this set in the future.

Bug: 837344
Change-Id: Id7f89cbe2cc6fdc92278a42ea033e0fef0baabcb
Reviewed-on: https://chromium-review.googlesource.com/1030906Reviewed-by: Jeremy Roman <jbroman@chromium.org>
Commit-Queue: Ian Clelland <iclelland@chromium.org>
Cr-Commit-Position: refs/heads/master@{#554482}

third_party/blink/renderer/platform/feature_policy/feature_policy.cc

@@ -151,9 +151,8 @@ bool IsSupportedInFeaturePolicy(mojom::FeaturePolicyFeature feature) {
     case mojom::FeaturePolicyFeature::kSyncXHR:
       return true;
     case mojom::FeaturePolicyFeature::kUnsizedMedia:
-      return RuntimeEnabledFeatures::FeaturePolicyExperimentalFeaturesEnabled();
     case mojom::FeaturePolicyFeature::kVerticalScroll:
-      return RuntimeEnabledFeatures::FeaturePolicyExperimentalFeaturesEnabled();
+      return RuntimeEnabledFeatures::ExperimentalProductivityFeaturesEnabled();
     default:
       return false;
   }
@@ -195,6 +194,14 @@ const FeatureNameMap& GetDefaultFeatureNameMap() {
       default_feature_name_map.Set(
           "picture-in-picture", mojom::FeaturePolicyFeature::kPictureInPicture);
     }
+    if (RuntimeEnabledFeatures::ExperimentalProductivityFeaturesEnabled()) {
+      default_feature_name_map.Set("sync-script",
+                                   mojom::FeaturePolicyFeature::kSyncScript);
+      default_feature_name_map.Set("unsized-media",
+                                   mojom::FeaturePolicyFeature::kUnsizedMedia);
+      default_feature_name_map.Set(
+          "vertical-scroll", mojom::FeaturePolicyFeature::kVerticalScroll);
+    }
     if (RuntimeEnabledFeatures::FeaturePolicyExperimentalFeaturesEnabled()) {
       default_feature_name_map.Set(
           "cookie", mojom::FeaturePolicyFeature::kDocumentCookie);
@@ -202,12 +209,6 @@ const FeatureNameMap& GetDefaultFeatureNameMap() {
           "domain", mojom::FeaturePolicyFeature::kDocumentDomain);
       default_feature_name_map.Set("docwrite",
                                    mojom::FeaturePolicyFeature::kDocumentWrite);
-      default_feature_name_map.Set("sync-script",
-                                   mojom::FeaturePolicyFeature::kSyncScript);
-      default_feature_name_map.Set("unsized-media",
-                                   mojom::FeaturePolicyFeature::kUnsizedMedia);
-      default_feature_name_map.Set(
-          "vertical-scroll", mojom::FeaturePolicyFeature::kVerticalScroll);
     }
     if (RuntimeEnabledFeatures::FeaturePolicyAutoplayFeatureEnabled()) {
       default_feature_name_map.Set("autoplay",

third_party/blink/renderer/platform/runtime_enabled_features.json5

@@ -425,6 +425,14 @@
       origin_trial_feature_name: "ExperimentalHardwareEchoCancellation",
       status: "experimental",
     },
+    // Enables a set of features intended to help improve web developer
+    // productivity, by restricting the use of potentially problematic web-
+    // platform behaviors, as well as adding new high-level APIs for common
+    // development patterns.
+    {
+      name: "ExperimentalProductivityFeatures",
+      status: "experimental"
+    },
     {
       name: "ExperimentalV8Extras",
       status: "experimental",