This change adds a new class, ChooserContextBase, that stores a list of
chosen objects for chooser-based permissions in website settings. It is
similar to PermissionContextBase.

UsbPermissionContext extends this and adds the concept of an ephemeral
device permission (for devices that can't be recorded in persistent
storage because they are not uniquely identifiable enough).

WebUSB is the initial consumer of this new API. WebUSBPermissionProvider
has been updated to check with the UsbPermissionContext and once there
is a chooser UI to populate the list of allowed devices the
--enable-webusb-on-any-origin flag can be removed.

BUG=529950

Review URL: https://codereview.chromium.org/1382783002

Cr-Commit-Position: refs/heads/master@{#359001}