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As part of the MD refresh, the WebUI button style changed dramatically.
We opted to reuse the paper-button and overriding the styling. The
button logic is fairly straightforward. That said, paper-button use
5 Polymer behaviors.

IronA11yKeysBehavior: a different way to handle keybindings.
IronButtonState: supports toggling and whether focus was from a
    keyboard.
IronControlState: handles disabling a control with the |disabled|
    property.
PaperButtonBehaviorImpl: supports button elevation based on the button
    state (focused, pressed, disabled and normal).
PaperRippleBehavior: lazily creates paper-ripple and forwards noink
    property to paper-ripple.

cr-button applies the MD refresh style directly and implements
only the functionality that is required. It makes use of the
PaperRippleBehavior and cr.ui.FocusOutlineManager (used in place of
IronButtonState which also supports toggling).

Bug: 967793
Change-Id: I0417974fcc69467be249920410b5ae77bdc529db
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1632894Reviewed-by: Demetrios Papadopoulos <dpapad@chromium.org>
Commit-Queue: Esmael El-Moslimany <aee@chromium.org>
Cr-Commit-Position: refs/heads/master@{#665213}