Subpixel rendering: Fieldset legend has a horizontal line through, when the fieldset is painted on odd device pixel position.
https://bugs.webkit.org/show_bug.cgi?id=132521
<rdar://problem/16803305>

Reviewed by Simon Fraser.

Use device pixel snapping when the fieldset's border gets clipped out for the legend's text. It ensures that
the device pixel snapped border gets properly clipped out.

Existing fieldset tests cover it.

* rendering/RenderFieldset.cpp:
(WebCore::RenderFieldset::paintBoxDecorations):


git-svn-id: http://svn.webkit.org/repository/webkit/trunk@168221 268f45cc-cd09-0410-ab3c-d52691b4dbfc