macOS: Update UA stylesheet for buttons, and remove border-adjustment behavior for -webkit-appearance:button

This CL makes the default size of CSS-rendered buttons same as
buttons with -webkit-appearance:button/push-button.

* Before this CL
UA stylesheet:
  padding: 2px 6px 3px 6px;
  border-width: 2px;

Style adjustment for -webkit-appearance:push-button:
  padding: 2px 8px 3px 8px; (adjusted by blink::ThemeMac)
  border-width: 0; (adjusted by blink::Theme)

Style adjustment for -webkit-appearance:button:
  padding: 2px 6px 3px 6px; (no adjustment)
  border-width: 0 2px 0 2px; (adjusted by blink::ThemeMac)

So, push-button and button had same sizes, and CSS fallback buttons
were taller than them.

* After this CL:
UA stylesheet:
  padding: 1px 7px 2px 7px;
  border-width: 1px;

Style adjustment for -webkit-appearance:push-button:
  padding: 2px 8px 3px 8px; (adjusted by blink::ThemeMac)
  border-width: 0; (adjusted by blink::Theme)

Style adjustment for -webkit-appearance:button:
  padding: 1px 7px 2px 7px; (no adjustment)
  border-width: 1px; (no adjustment)

As for push-button and button, their size are not changed by this CL,
and the size of CSS fallback buttons matches to them.

In general, style adjustment by theme code should be reduced in order
to improve platform predictability.

This CL affects only macOS.

Bug: 546679
Change-Id: I4f10d546bf01851ff8ff1c6679bd4a1a56ea474a
Reviewed-on: https://chromium-review.googlesource.com/590392


Commit-Queue: Kent Tamura <tkent@chromium.org>
Reviewed-by: Keishi Hattori <keishi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#490390}