Fix clicking area below the scrollbar thumbs in Files app.

Fix clicking area below the scrollbar thumbs in Files app.

Before, the entire vertical bar was accepting clicks, so it was hard to select
items in grid view when starting from far right.

TEST=Tested manually by trying to click around the scrollbar thumb (button).
BUG=461728
Committed: https://crrev.com/5a16e5bdf5d7fdad975cc2bef27caecaebe93c36
Cr-Commit-Position: refs/heads/master@{#318006}

[mtomasz, 2015-02-25 08:47:30]

mtomasz@chromium.org changed reviewers:
+ fukino@chromium.org

[mtomasz, 2015-02-25 08:47:38]

@fukino: PTAL. Thanks.

[fukino, 2015-02-25 09:06:21]

lgtm. Thank you!

[mtomasz, 2015-02-25 09:06:58]

The CQ bit was checked by mtomasz@chromium.org

[commit-bot: I haz the power, 2015-02-25 09:07:56]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/956843002/1

[commit-bot: I haz the power, 2015-02-25 09:42:52]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-02-25 09:43:28]

Patchset 1 (id:??) landed as
https://crrev.com/5a16e5bdf5d7fdad975cc2bef27caecaebe93c36
Cr-Commit-Position: refs/heads/master@{#318006}