Fix selection of label having control element.

Any click event on label goes to its control element. So, if
we tried to select the text inside label by dragging the mouse
or by double clicking on the text, it went to its control
element. This patch checks whether there is selection or not,
after the click is happened. If there is selection then do
nothing else continue.

New Layout test added:
 fast/forms/label/label-selection.html

BUG=148912

Review URL: https://codereview.chromium.org/258933006

git-svn-id: svn://svn.chromium.org/blink/trunk@173310 bbb929c8-8fbe-4397-9dbb-9b2b20218538