Enable Permissions-Policy header by default.

This enables the detection and parsing of the "Permissions-Policy" HTTP
response header, but does not remove support for the "Feature-Policy"
header. If both headers are returned with a document, then any feature
mentioned in both headers will use the allowlist specified by
Permissions-Policy. A console message will be emitted in that case, to
alert developers to the potential conflict.

Intent-to-Ship thread:
https://groups.google.com/a/chromium.org/d/msgid/blink-dev/CAOMQ%2Bw-hVfxO8YYX_WAKNCD96sg_bnFmTDbYDsaL4pfw3s9f4A%40mail.gmail.com

Bug: 1095641

Change-Id: I02f9eb34405bf3c842705bf2c18a14b62fa076e6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2530793
Commit-Queue: Ian Clelland <iclelland@chromium.org>
Reviewed-by: Yaron Friedman <yfriedman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#826436}

content/public/common/content_features.cc

@@ -449,7 +449,7 @@ const base::Feature kPeriodicBackgroundSync{"PeriodicBackgroundSync",
 
 // Enable permission policy for configuring and restricting feature behavior.
 const base::Feature kPermissionsPolicyHeader{"PermissionsPolicyHeader",
-                                             base::FEATURE_DISABLED_BY_DEFAULT};
+                                             base::FEATURE_ENABLED_BY_DEFAULT};
 
 // If Pepper 3D Image Chromium is allowed, this feature controls whether it is
 // enabled.

third_party/blink/renderer/platform/runtime_enabled_features.json5

@@ -1520,7 +1520,7 @@
     },
     {
       name: "PermissionsPolicyHeader",
-      status: "experimental"
+      status: "stable"
     },
     {
       name: "PermissionsRequestRevoke",