[A11y] Update CSS anchors only when necessary

CSS anchors in the AXObject cache were previously updated on every
layout. This is unnecessary; updates are only required when anchors are
present on the page.

CSSAnchorChanged() is moved to UpdatePositionVisibilityAfterLayout(),
which exits early if no anchor query exists.

Bug: 386996729
Change-Id: If27bd3b7ba3d00cacef23349fe4514260f9f765e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6209252
Reviewed-by: Koji Ishii <kojii@chromium.org>
Commit-Queue: Jocelyn Tran <jocelyntran@google.com>
Cr-Commit-Position: refs/heads/main@{#1412876}