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This CL introduces the 'gaia-icon-button' decorator, which can be applied to any
<button>.
All icon buttons introduced recently are <button is="gaia-icon-button">s now.

BUG=478545,472671
TEST=manual

Review URL: https://codereview.chromium.org/1133733003

Cr-Commit-Position: refs/heads/master@{#329284}