Rollback ContextMenu

This CL aims to drop the context menu feature entirely.
<menuitem> tag can be no longer used and <menu type='context'>, <menu label='...'> are no longer supported.

BUG=87553

Review-Url: https://codereview.chromium.org/2905763003
Cr-Commit-Position: refs/heads/master@{#476230}
68 files changed