Use click event for search field cancel button

The previous behavior was to select all on mouse down and clear on mouse up. This matched the Cocoa NSSearchField behavior.
On Android the button may be so small users couldn't tap it reliably and only mousedown would fire.

BUG=329326

Review URL: https://codereview.chromium.org/93863007

git-svn-id: svn://svn.chromium.org/blink/trunk@170202 bbb929c8-8fbe-4397-9dbb-9b2b20218538