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This CL changes the back button of a detailed view to have custom shape
following the UX spec.

To do this, this CL splits CustomShapeButton from CollapseButton, and
derives CollapseButton and BackButton from CustomShapeButton.

Screenshot: http://screen/Lkuj2DJCYHP
UX spec: http://shortn/_N5oGsw6QZ0

TEST=manual
BUG=850396

Change-Id: I08554db7e78e710e96dd670917d612dbadee5ccb
Reviewed-on: https://chromium-review.googlesource.com/1096813Reviewed-by: Yoshiki Iguchi <yoshiki@chromium.org>
Commit-Queue: Tetsui Ohkubo <tetsui@chromium.org>
Cr-Commit-Position: refs/heads/master@{#567101}