null check for GetLastActive in NotifyThemeChanged.
null check for GetLastActive in NotifyThemeChanged.

BUG=chromium-os:13082
TEST=none
Review URL: http://codereview.chromium.org/6688011

git-svn-id: http://src.chromium.org/svn/trunk/src@78637 4ff67af0-8c30-449e-8e8b-ad334ec8d88c

Former-commit-id: 51bf22d03d8eb1d937cefe687833d1a1caacfe2e
meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser