Clicking on a non-link A element in a LABEL element should activate the associated control.

'A' element without href attribute should not be an interactive content.

BUG=339629
TEST=automated

Review URL: https://codereview.chromium.org/144243006

git-svn-id: svn://svn.chromium.org/blink/trunk@166595 bbb929c8-8fbe-4397-9dbb-9b2b20218538