MD Bookmarks: Enable the delete button in the toolbar overlay

The Delete button in the overlay which appears when selecting more than
one item will now delete all selected items when clicked.

BUG=692827
CQ_INCLUDE_TRYBOTS=master.tryserver.chromium.linux:closure_compilation

Review-Url: https://codereview.chromium.org/2893833002
Cr-Commit-Position: refs/heads/master@{#473835}
10 files changed