Revert "Remove !important from <select> color rules"

This reverts commit 6fdfd916b7f4ae574ea634dbab6536080633d6f4.

Reason for revert: Seems to be causing some WebUI issues (crbug.com/1244003 and crbug.com/1244535).

Original change's description:
> Remove !important from <select> color rules
>
> These were added in [1] in response to a bug [2] that was essentially
> "you can style the checked option so that it is the same color as the
> other options". And while that's true, I'm not sure why that's a good
> reason to outlaw changes to color/background-color entirely. As
> pointed out in comment [3] there, this CSS today would cause the
> same issue:
>
>   option {
>     background: rgb(206, 206, 206);
>   }
>
> because it would match the existing :checked UA style. And because
> of the !important rules, there would be no way to change the :checked
> style to ensure contrast.
>
> [1] https://src.chromium.org/viewvc/blink?revision=179782&view=revision
> [2] https://crbug.com/398417
> [3] https://bugs.chromium.org/p/chromium/issues/detail?id=398417#c8
>
> Fixed: 562990
> Bug: 398417
> Change-Id: Ic2bf2eaebbe2d736c047e404b855acdcf9c264ba
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3119649
> Reviewed-by: Joey Arhar <[email protected]>
> Commit-Queue: Mason Freed <[email protected]>
> Cr-Commit-Position: refs/heads/main@{#915641}

Bug: 398417
Change-Id: I2ade5e6b5faa164a3dce278eb5581f816f36d25c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3130361
Bot-Commit: Rubber Stamper <[email protected]>
Commit-Queue: Mason Freed <[email protected]>
Reviewed-by: Joey Arhar <[email protected]>
Cr-Commit-Position: refs/heads/main@{#916451}