Set colors for default buttons when disabled to match Chrome OS colors.

Adds color variable '@insensitive_text_color' set to #80868b
(from Chrome spec).

For non-action ('.suggested-action', '.destructive-action') buttons in
the ':disabled' state:
- Set color to '@insensitive_text_color' (without transparency).
- Set background-color to '@theme_base_color' (without transparency).
- 'border-color: @borders;' (for rules that set border-color).
- 'border: 1px solid @borders;' (for rules that set border).
Other border colors are left unchanged.

The following mods were used to construct these changes using the CSS
rewriter from https://chromium-review.googlesource.com/1134635

Mod(r'button.*:disabled',
    anti=[r'-action'],
    remove=color(),
    replace=r'\n  color: @insensitive_text_color;',
    enabled=True),
Mod(r'button.*:disabled',
    anti=[r'-action'],
    remove=background_color(),
    replace=r'\n  background-color: @theme_base_color;',
    enabled=True),
Mod(r'button.*:disabled',
    anti=[r'-action'],
    remove=border_color(),
    replace=r'\n  border-color: @borders;',
    enabled=True)

Bug: 822495
Change-Id: I7125bb689be02d5bae44fae30ed10f15086a687f
Reviewed-on: https://chromium-review.googlesource.com/1132830
Commit-Ready: Josh Pratt <jopra@chromium.org>
Tested-by: Josh Pratt <jopra@chromium.org>
Reviewed-by: Raymes Khoury <raymes@chromium.org>
6 files changed