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When no arrow button shows, the app icons are chopped under RTL. The
reason is that |available_space_| is not adapted to RTL. The issue
is not reproducible under overflow mode. Because in overflow mode,
|available_space_| happens to be the same with the mirrored one.

This CL fixes such an issue by mirroring |available_space_| for the
horizontal shelf under RTL.

Bug: 1016271
Change-Id: I76ec521048b34da9fad97b8d0af6bfd3b8ebb327
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1872800
Commit-Queue: Andrew Xu <andrewxu@chromium.org>
Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#707978}