Handle selection changing via keyboard and mouse drag.

IsChanging only is called during mouse selection; DidChange is called during keyboard selections too.

BUG=319708
TEST=as in bug

Review URL: https://codereview.chromium.org/76493002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@236594 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 236594