Add the UserAgentReduction runtime feature for the Origin Trial

The UserAgentReduction runtime feature gates the code for the origin
trial intended to send reduced user-agent strings to the site.

The web feature was added in crrev.com/c/3002633.

Chrome Platform Status: https://chromestatus.com/feature/5704553745874944
Design Doc: https://docs.google.com/document/d/1feIxK9S7oNgT2oGGebbxE9X0O-4wTKcsP_gRaY99tq4

Bug: 1222742
Change-Id: I6061623b46a6cc78f716c2a88dc7fdcc3614479d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3057751
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Commit-Queue: Ali Beyad <abeyad@chromium.org>
Cr-Commit-Position: refs/heads/master@{#906203}

third_party/blink/renderer/platform/runtime_enabled_features.json5

@@ -2211,6 +2211,11 @@
       name: "UserAgentClientHint",
       status: "stable",
     },
+    {
+      name: "UserAgentReduction",
+      origin_trial_feature_name: "UserAgentReduction",
+      status: "experimental",
+    },
     {
       name: "UUID",
       status: "stable"