Fix Aura tooltip sizing and layout.

Fix Aura tooltip sizing and layout.

r296008 regressed TooltipAura vertical placement.
(that change removed vertical centering of multi-line text)
(TooltipAura implicitly used this for vertical padding)

Add an actual border and explicit Label sizing instead.
(this allocates space for the padding in the Label border)
(it also base-line centers the text in the correct bounds)

See before/after pics at http://crbug.com/417602#c5

BUG=417602
TEST=Chrome OS and "Windows 8 mode" Chrome tooltips look good.
R=sky@chromium.org

Committed: https://crrev.com/a23b072d5ecf5020d513a6d432cf9cfc080ea179
Cr-Commit-Position: refs/heads/master@{#296874}

[msw, 2014-09-26 02:11:00]

msw@chromium.org changed reviewers:
+ sky@chromium.org

[msw, 2014-09-26 02:14:03]

Hey Scott, please take a look; thanks!

[sky, 2014-09-26 03:35:21]

LGTM

[msw, 2014-09-26 03:46:06]

The CQ bit was checked by msw@chromium.org

[commit-bot: I haz the power, 2014-09-26 03:46:30]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/602313002/1

[commit-bot: I haz the power, 2014-09-26 03:50:57]

Committed patchset #1 (id:1) as 9553db2ad14a802d7208c50ff3adcee6d93e43c3

[commit-bot: I haz the power, 2014-09-26 03:51:41]

Patchset 1 (id:??) landed as
https://crrev.com/a23b072d5ecf5020d513a6d432cf9cfc080ea179
Cr-Commit-Position: refs/heads/master@{#296874}