[DevTools] Minor tweak in popover positioning.

[DevTools] Minor tweak in popover positioning.

When positioning popover below anchor, the visible bottom coordinate was wrong: |arrowHeight - borderWidth| instead of |borderRadius|.
Consequently, it caused wrong position update, and popover did extend below container element, which is unacceptable and leads to bad layout.

BUG=363535
Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=172281

[dgozman, 2014-04-22 16:08:38]

Could you please take a look?

[apavlov, 2014-04-22 16:18:03]

lgtm

[dgozman, 2014-04-22 16:18:42]

The CQ bit was checked by dgozman@chromium.org

[commit-bot: I haz the power, 2014-04-22 16:18:57]

CQ is trying da patch. Follow status at
https://chromium-status.appspot.com/cq/dgozman@chromium.org/246823003/1

[commit-bot: I haz the power, 2014-04-22 17:26:56]

The CQ bit was unchecked by commit-bot@chromium.org

[commit-bot: I haz the power, 2014-04-22 17:26:57]

Try jobs failed on following builders:
  tryserver.blink on linux_blink_dbg

[dgozman, 2014-04-22 17:33:40]

The CQ bit was checked by dgozman@chromium.org

[commit-bot: I haz the power, 2014-04-22 17:33:55]

CQ is trying da patch. Follow status at
https://chromium-status.appspot.com/cq/dgozman@chromium.org/246823003/1

[commit-bot: I haz the power, 2014-04-22 18:39:59]

The CQ bit was unchecked by commit-bot@chromium.org

[commit-bot: I haz the power, 2014-04-22 18:40:00]

Try jobs failed on following builders:
  tryserver.blink on linux_blink_dbg

[dgozman, 2014-04-22 19:48:21]

The CQ bit was checked by dgozman@chromium.org

[commit-bot: I haz the power, 2014-04-22 19:48:42]

CQ is trying da patch. Follow status at
https://chromium-status.appspot.com/cq/dgozman@chromium.org/246823003/1

[commit-bot: I haz the power, 2014-04-22 23:35:37]

Change committed as 172281