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This shows the same toolbar overlay as MD History, with 'Cancel'/'Delete'
options. In MD Bookmarks, the overlay is only shown when more than one
item is selected, to avoid flickering when double clicking items.

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

Review-Url: https://codereview.chromium.org/2827573002
Cr-Commit-Position: refs/heads/master@{#467582}