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This removes a number of CSS mixins which allowed almost all parts of
the toolbar to be changed. Instead, this adds better support to the
toolbar for having an action button on the right side of the element,
and migrates History and Downloads to use it.

BUG=665700
CQ_INCLUDE_TRYBOTS=master.tryserver.chromium.linux:closure_compilation

Review-Url: https://codereview.chromium.org/2562393003
Cr-Commit-Position: refs/heads/master@{#439710}