Never ignore HTMLLabelElement's click() calls

Programatic HTMLElement::click() is expected to execute the activation
behavior of the element. For label element, it should always trigger
the target control's click behavior even if the label is selected.

BUG=375358
TEST=fast/events/label-click.html

Review URL: https://codereview.chromium.org/291243002

git-svn-id: svn://svn.chromium.org/blink/trunk@174613 bbb929c8-8fbe-4397-9dbb-9b2b20218538