pin-infobar: add to browser window features

This change makes the pin-to-taskbar infobar a member of
BrowserWindowFeatures, i.e., owned by a browser window. It adds a new
method, `MaybeShowInfoBarForBrowser()`, to be called when infobars are
added during startup.

With this change, the pin-to-taskbar infobar will be shown only if the
default-browser infobar wasn't already shown in the session. If the
pin-to-taskbar infobar is shown, the default-PDF-viewer infobar won't be
shown. This is enforced because these infobars ask similar questions,
and showing more than one in a session is too much.

This change also adds tests that ensure the pin-to-taskbar infobar won't
show if a previous infobar was shown.

All of the above behavior is behind a disabled-by-default experiment.

Bug: 420960161
Change-Id: Ibbc9874ef0888b02beb98e080a7518ebf8761f80
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6701409
Commit-Queue: Jesse McKenna <jessemckenna@google.com>
Reviewed-by: Darryl James <dljames@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1484515}