Un-delete bringing windows to front during tab dragging.

Once upon a time, dragging a tab over a window in the background (not
its tabstrip - the contents) would, after a short pause, bring the
window to the front. This made it easier to drag tabs between windows on
crowded desktops.

This was disabled in https://crrev.com/c/3905542 and then deleted in
https://crrev.com/c/5321488 . I think it was disabled in error - the
feature still works (at least on Mac), and it's a good QoL polish
feature for tab dragging.

This CL is just a revert of the relevant parts of the above linked CLs,
with a couple of style fixes. I would love to include a regression test,
but the tests are too broken on my dev machine (macOS).

Low-Coverage-Reason: HARD_TO_TEST
Change-Id: I0e4aaa6a892533474637dccf259a7305027a263f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6051014
Reviewed-by: Keren Zhu <kerenzhu@chromium.org>
Commit-Queue: Taylor Bergquist <tbergquist@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1390731}