MD Bookmarks: Pull context menu into separate element
This creates a new element, <bookmarks-command-manager>, which is
responsible for showing the context menu, hiding commands which are
unavailable and responding to command clicks. Pulling out the context
menu like this will make it easier to reuse in multiple places, and
make it simpler to add keyboard shortcuts for menu items.
BUG=692827
CQ_INCLUDE_TRYBOTS=master.tryserver.chromium.linux:closure_compilation
Review-Url: https://codereview.chromium.org/2834493006
Cr-Commit-Position: refs/heads/master@{#469592}
12 files changed
