Change sizing of views::Separator

Change sizing of views::Separator

Using the allocated height/width in GetPreferredSize is a bit hokey and
could potentially lead to unexpected behavior. This also fixes the use
of borders on views::Separators.

See also https://codereview.chromium.org/1270783005/

BUG=none
TBR=sky@chromium.org

Committed: https://crrev.com/5612b1032ded056a736d7877dc9748ecb47bcc9b
Cr-Commit-Position: refs/heads/master@{#341952}

[Evan Stade, 2015-08-05 18:56:58]

The CQ bit was checked by estade@chromium.org

[Evan Stade, 2015-08-05 18:57:10]

The CQ bit was unchecked by estade@chromium.org

[Evan Stade, 2015-08-05 18:57:11]

The CQ bit was checked by estade@chromium.org

[commit-bot: I haz the power, 2015-08-05 18:58:36]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1276613002/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1276613002/1

[commit-bot: I haz the power, 2015-08-05 20:14:26]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-08-05 20:15:07]

Patchset 1 (id:??) landed as
https://crrev.com/5612b1032ded056a736d7877dc9748ecb47bcc9b
Cr-Commit-Position: refs/heads/master@{#341952}

[sky, 2015-08-05 21:38:26]

LGTM