Populate emoji bar after PickerView is added to widget.

When items are added to the Picker emoji bar, the preferred size of
emoji bar views is used to determine how many items can be added without
overflowing the emoji bar.

Currently, Picker populates its emoji bar (with recent emojis etc.) when
PickerView is being created. This occurs before PickerView is added to a
widget, at which point the gif button preferred size may not be correct.
Then, too many emoji items can be added and the gif button can end up
truncated.

To fix this, instead populate the emoji bar after PickerView is added to
the Picker widget (after preferred sizes have been correctly computed).

As drive by, also simplify ResetEmojiBarToZeroState (since delegate_ is
guaranteed to be non-null) and prevent the gif button from eliding (for
better fallback behaviour if a similar truncation bug arises).

Bug: b:355544058
Change-Id: I99bd501a31246f61840036eaab6fb4cd52d91b6d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5751545
Reviewed-by: Darren Shen <shend@chromium.org>
Commit-Queue: Michelle Chen <michellegc@google.com>
Cr-Commit-Position: refs/heads/main@{#1335841}