MD WebUI: Pull 'X selected' toolbar overlay from History into shared element

The overlay is designed for use with cr-toolbar, providing an overlay
with the number of items selected, and a 'cancel' and 'delete' button.
This new shared element will be used by MD Bookmarks.

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

Review-Url: https://codereview.chromium.org/2800603002
Cr-Commit-Position: refs/heads/master@{#463530}
11 files changed