Added a unittest for compositor observers.
Added a unittest for compositor observers.

Passes with both the browser and WebKit compositor.

BUG=None
TEST=compositor_unittests

Review URL: http://codereview.chromium.org/8538018

git-svn-id: de016e52bd170d2d4f2344f9bf92d50478b649e0@110242 0039d316-1c4b-4281-b951-d872f2087c98

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