Cleanup: Pull out Observer class from AppListView.

The search:

"class Observer {" package:^chromium$ file:(^src/ui/app_list/)

in cs.chromium.org

results in
https://code.google.com/p/chromium/codesearch#chromium/src/ui/app_list/views/app_list_view.h&l=36

So we should move that into app_list_view_observer.h

BUG=322583
TEST=None
R=xiyuan@chromium.org
TBR=sky

Review URL: https://codereview.chromium.org/106163006

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@239338 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 239338