Tapping <select multiple> option toggles selection

We want to be able to use <select multiple> with only touch.
Mouse behavior won't change.

BUG=356499

Review URL: https://codereview.chromium.org/211093004

git-svn-id: svn://svn.chromium.org/blink/trunk@170111 bbb929c8-8fbe-4397-9dbb-9b2b20218538