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This CL adds support for fading out text of LabelButton using
AnimatingLayoutManager. Concretely, when ShrinkDownThenClearText() gets
called, the button reports a new desired size without it's label. The
AnimatingLayoutManager can then resize the button using an animation
to its newly desired size while the label is still (partially) visible.
Once the animation is finished, the button clears its label.

This CL directly makes use of this functionality for ToolbarButton.

Bug: 1002160
Change-Id: Id2569d4c58497ccb3198ca70a01cc2d7eb270a2f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1810977
Commit-Queue: Jan Krcal <jkrcal@chromium.org>
Reviewed-by: Peter Boström <pbos@chromium.org>
Cr-Commit-Position: refs/heads/master@{#699252}