[MD settings] edit content settings exceptions

This CL enables the edit dialog for content settings exceptions beyond
the cookies category. Specifically: Automatic Downloads, Background
Sync, Images, JavaScript, Notifications, Plugins, Popups, and
Unsandboxed Plugins. The intention is to match which exceptions are
editable in the old options.

BUG=695578
CQ_INCLUDE_TRYBOTS=master.tryserver.chromium.linux:closure_compilation

Review-Url: https://codereview.chromium.org/2749023008
Cr-Commit-Position: refs/heads/master@{#458258}
6 files changed