[iOS] Use LayoutGuideCenter in ToolbarButton

This CL hooks up LayoutGuideCenter to ToolbarButton. It also adds an
extra early return in LayoutGuideCenter, so ToolbarButton's repeated
calls to `reference(view:under:)` don't cause needless work to be done.

Bug: 1344522
Change-Id: Ic61106a900eff81385ee87dae3580382e54ace73
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3762692


Reviewed-by: Mark Cogan <marq@chromium.org>
Commit-Queue: Robbie Gibson <rkgibson@google.com>
Reviewed-by: Louis Romero <lpromero@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1026909}