MediaPermission is the interface for media code to check/request media related
permissions. MediaPermissionDispatcher is a MediaPermission implementation in
content/.

Plan of follow-up CL:
- Use MediaPermission in ProxyDecryptor and remove current protected media
  identifier info bar code in the browser process (on Android and ChromeOS).

BUG=446263
TEST=Tested on Android with encrypted-media-requestmediakeysystemaccess.html
layout test by calling MediaPermission methods manually in
webencryptedmediaclient_impl.cc.

Review URL: https://codereview.chromium.org/871663003

Cr-Commit-Position: refs/heads/master@{#313303}