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This CL adds arrow-key navigation to the main history list and makes a
single history item tab-navigable by using cr.ui.FocusRow and
iron-list's tab index property.

BUG=623011
CQ_INCLUDE_TRYBOTS=master.tryserver.chromium.linux:closure_compilation

Review-Url: https://codereview.chromium.org/2334553002
Cr-Commit-Position: refs/heads/master@{#419956}