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This CL removes the global .secondary-button class. At one point we had
several types of buttons, but we eventually narrowed down to two buttons:
.primary-button and .secondary-button; though some buttons were not
specified as either. That meant we effectively had three button classes.

Rather than add .secondary-button to each paper-button that isn't
explicitly secondary already, I've removed that class. Now there are
.primary-buttons and normal buttons; i.e. any non-primary-button is a
secondary button (aka normal) button.

Note: The references to .secondary-button are still in place (based on
Steven's review comment below) and there are a couple places where a
.secondary-button is used locally within a dom-module; those have been
left unchanged.

BUG=721268
CQ_INCLUDE_TRYBOTS=master.tryserver.chromium.linux:closure_compilation

Review-Url: https://codereview.chromium.org/2889493002
Cr-Commit-Position: refs/heads/master@{#472301}