Cleanup upstreamed Android infobar code.

Some simplifications, and a variety of minor style changes to become consistent
with all the rest of the infobar code.

BUG=none
TEST=none
R=miguelg@chromium.org

Review URL: https://codereview.chromium.org/24562006

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@225282 0039d316-1c4b-4281-b951-d872f2087c98