Add NULL check for NativeTheme in scrollbar methods. A NULL theme can

Add NULL check for NativeTheme in scrollbar methods. A NULL theme can
happen with some code paths.

BUG=175600
TEST=none
R=sadrul@chromium.org


Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=182142

[sadrul, 2013-02-12 20:51:16]

LGTM

[commit-bot: I haz the power, 2013-02-12 22:14:57]

CQ is trying da patch. Follow status at
https://chromium-status.appspot.com/cq/sky@chromium.org/12212144/1

[commit-bot: I haz the power, 2013-02-13 06:27:57]

Change committed as 182142