Fix crash during ChromeOS login
Fix crash during ChromeOS login

Add NULL check before dispatching brightness-changed evnnt

BUG=chromium-os:26852
TEST=login success

Review URL: https://chromiumcodereview.appspot.com/9604001

git-svn-id: de016e52bd170d2d4f2344f9bf92d50478b649e0@125098 0039d316-1c4b-4281-b951-d872f2087c98

gavinp/chromium,gavinp/chromium,adobe/chromium,adobe/chromium,gavinp/chromium,ropik/chromium,gavinp/chromium,yitian134/chromium,adobe/chromium,yitian134/chromium,adobe/chromium,ropik/chromium,gavinp/chromium,yitian134/chromium,ropik/chromium,ropik/chromium,gavinp/chromium,ropik/chromium,adobe/chromium,adobe/chromium,adobe/chromium,gavinp/chromium,adobe/chromium,yitian134/chromium,yitian134/chromium,gavinp/chromium,yitian134/chromium,adobe/chromium,gavinp/chromium,yitian134/chromium,ropik/chromium,yitian134/chromium,gavinp/chromium,yitian134/chromium,yitian134/chromium,ropik/chromium,adobe/chromium,ropik/chromium,ropik/chromium,adobe/chromium