WebUI: Remove is="custom-style" attributes from most places.

Basically removing it from everywhere where is not used by OOBE.

Bug: 944281
Change-Id: I4fd20800c38dbec929612b81442642dc9f39fb2b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1585230
Commit-Queue: Demetrios Papadopoulos <dpapad@chromium.org>
Auto-Submit: Demetrios Papadopoulos <dpapad@chromium.org>
Reviewed-by: Rebekah Potter <rbpotter@chromium.org>
Cr-Commit-Position: refs/heads/master@{#654289}
12 files changed