Revert of Revert of Fix selection of label having control element. (https://codereview.chromium.org/290153002/)

Reason for revert:
The previously reverted behavior is actually correct and what we wanted.

BUG=148912,370859

Original issue's description:
> Revert of Fix selection of label having control element. (https://codereview.chromium.org/258933006/)
> 
> Reason for revert:
> Causes bug 370859
> 
> BUG=148912,370859
> TBR=keishi
> 
> Original issue's description:
> > 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
> > 
> > Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=173310
> 
> Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=174193

TBR=tkent@chromium.org,keishi@chromium.org,deepak.sa@samsung.com
NOTREECHECKS=true
NOTRY=true
BUG=148912,370859

Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=174202

Review URL: https://codereview.chromium.org/282343005

git-svn-id: svn://svn.chromium.org/blink/trunk@174296 bbb929c8-8fbe-4397-9dbb-9b2b20218538