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This is a quick fix for the bug on Mac that an activatable bubble
initially appears in inactive style even when it's in an active browser
window.

We generally want the inactive/active visual style to be in sync for all
widgets in a widget tree. Previously, PaintAsActiveLock partially
achieves this goal by having bubble widget lock its parent widget.
This solution has two caveats:

a) The lock is only on the parent widget, but not on other ancestors.
   If we have a password bubble with an focused in-product help promo,
   the password bubble will be in active style, but the browser window
   will be in inactive style.
b) The lock does not lock sibling widgets.
   This is the culprit of crbug/1112244 and is addressed by this CL,
   but only for LabelButton (which is the only impacted control).

A holistic solution that locks all widgets in tree is WIP to address
both caveats.

Bug: 1112244
Change-Id: I230e23d6880f44ab9b569fc1d57fb223d6a01ea1
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2508447
Commit-Queue: Keren Zhu <kerenzhu@chromium.org>
Reviewed-by: Peter Kasting <pkasting@chromium.org>
Cr-Commit-Position: refs/heads/master@{#822984}