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Notification center used fixed font sizes for UI labels. This is not the
right way of settings font size.

This CL
- removes ash::message_center_style::GetFontListForSizeAndWeight().
- replaces all fixed font sizes with delta.

TEST=manual - compared screenshots and verified it cause no visual
change on actual device (chell)
BUG=793207

Change-Id: Ia2c50965e6a132520d63921252e78a76a644734f
Reviewed-on: https://chromium-review.googlesource.com/817456Reviewed-by: Evan Stade <estade@chromium.org>
Commit-Queue: Tetsui Ohkubo <tetsui@chromium.org>
Cr-Commit-Position: refs/heads/master@{#523696}