Enable chrome.metricsPrivate for the untrusted context of MediaApp so we can use UMA.

Doc: http://go/backlight-ea-uma-metrics (slightly outdated)

Bug: 996088, b/128567301
Change-Id: Ia272a35815ac92de954a6e5c43e1c2d4ea502e0d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2000440Reviewed-by: Trent Apted <tapted@chromium.org>
Reviewed-by: Ben Wells <benwells@chromium.org>
Commit-Queue: David Lei <dlei@google.com>
Cr-Commit-Position: refs/heads/master@{#733884}

extensions/common/api/_api_features.json

@@ -309,6 +309,7 @@
     "matches": [
       "chrome://bookmarks/*",
       "chrome://extensions/*",
+      "chrome://media-app-guest/*",
       "chrome://os-settings/*",
       "chrome://settings/*",
       "chrome://sync-confirmation/*",