Added methods to SystemTray needed for new first-run UI.

* Added method for opening system bubble in persistent state.
* Added method for retrieving view of help button.

TEST=ash_unittests --gtest_filter=SystemTrayTest.PersistentBubble
BUG=269286

Review URL: https://codereview.chromium.org/38713002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@230652 0039d316-1c4b-4281-b951-d872f2087c98
Blink roll 160353:160404

http://build.chromium.org/f/chromium/perf/dashboard/ui/changelog_blink.html?url=/trunk&range=160354:160404&mode=html
TBR=
BUG=

Review URL: https://codereview.chromium.org/39153002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@230653 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 230652
SVN changes up to revision 230653