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This adds a new widget, ButtonCompat, which is a Material-styled button.
On pre-L devices, the widget approximates the Material style, but
doesn't have ripples or shadows. The color of ButtonCompat can be easily
changed using setButtonColor().

This also adds a style ButtomBorderlessCompat than can be applied to
Buttons to make them look like borderless Material buttons.

BUG=442690

Review URL: https://codereview.chromium.org/902893004

Cr-Commit-Position: refs/heads/master@{#314942}