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- Expand/Collapse button should be in the fixed size.
- Secure minumum gap between Expand/Collapse and other buttons.
- Shring gap between other buttons when there's not enough space

Bug: 847100
Change-Id: I295126c63ed2702ca7e10906f6d4ca599485e904
Reviewed-on: https://chromium-review.googlesource.com/1078031
Commit-Queue: Tatsuhisa Yamaguchi <yamaguchi@chromium.org>
Reviewed-by: Yoshiki Iguchi <yoshiki@chromium.org>
Reviewed-by: Tetsui Ohkubo <tetsui@chromium.org>
Cr-Commit-Position: refs/heads/master@{#563122}