Fix background of overflow shelf icon

Fix background of overflow shelf icon

BUG=640642
TEST=Add shelf icons and/or increase zoom until shelf overflows.
Observe that background of overflow button is square, not round.

Review-Url: https://codereview.chromium.org/2277523004
Cr-Commit-Position: refs/heads/master@{#414501}
(cherry picked from commit cc77d07303f42212610e01d7fac2e8e4dd01647a)

Committed: https://chromium.googlesource.com/chromium/src/+/805f2f810a2ab4bf8380b61571d79c7788c5044c

[Greg Levin, 2016-08-26 22:05:57]

Description was changed from

==========
Fix background of overflow shelf icon

BUG=640642
TEST=Add shelf icons and/or increase zoom until shelf overflows.
Observe that background of overflow button is square, not round.

Review-Url: https://codereview.chromium.org/2277523004
Cr-Commit-Position: refs/heads/master@{#414501}
(cherry picked from commit cc77d07303f42212610e01d7fac2e8e4dd01647a)
==========

to

==========
Fix background of overflow shelf icon

BUG=640642
TEST=Add shelf icons and/or increase zoom until shelf overflows.
Observe that background of overflow button is square, not round.

Review-Url: https://codereview.chromium.org/2277523004
Cr-Commit-Position: refs/heads/master@{#414501}
(cherry picked from commit cc77d07303f42212610e01d7fac2e8e4dd01647a)

Committed:
https://chromium.googlesource.com/chromium/src/+/805f2f810a2ab4bf8380b61571d7...
==========

[Greg Levin, 2016-08-26 22:05:59]

Committed patchset #1 (id:1) manually as
805f2f810a2ab4bf8380b61571d79c7788c5044c.