Fix tinting of bottom bar button icons.

This CL fixes two issues with tinting of bottom bar button icons:

1. The tint colour was not chosen in the same way the top bar icon tint colour was chosen, which could lead to inconsistent colour on top and bottom bars. See e.g.:

Before:  https://screenshot.googleplex.com/9twp6KJjn9qNCR9.
After: https://screenshot.googleplex.com/ALYzjx98xgxAJCS

2. A side-effect of (1) was that the tint was not chosen based on whether the embedder-provided background colour was dark or light, but rather based on whether the current theme was dark or light. This could lead to cases like this, where e.g. embedder has provided a lighter colour for dark theme:

Before: https://screenshot.googleplex.com/kptHgDvYcD4yjek.
After: https://screenshot.googleplex.com/6XCojfCTXsYuRTf

Bug: 381508007
Change-Id: I144635f40b9fdf0c3a7725d9ccc52c799ae12811
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6054788
Reviewed-by: Jinsuk Kim <jinsukkim@chromium.org>
Commit-Queue: Edmund Wright <edmundw@google.com>
Auto-Submit: Edmund Wright <edmundw@google.com>
Code-Coverage: findit-for-me@appspot.gserviceaccount.com <findit-for-me@appspot.gserviceaccount.com>
Reviewed-by: Sinan Sahin <sinansahin@google.com>
Reviewed-by: Theresa Sullivan <twellington@chromium.org>
Commit-Queue: Theresa Sullivan <twellington@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1391107}