Clean up PPAPITest infobar handling.
(1) Replace list of expectations with a single one, since we're only ever
    expecting one infobar at most.
(2) Close the infobar after acting on it, so it can clean up properly.
(3) Move infobar handling and closure to an async function, so that (2) is
    possible without deleting any objects other observers might care about.

BUG=62154
TEST=none
R=yzshen@chromium.org

Review URL: https://codereview.chromium.org/83073014

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@237116 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 237116