REGRESSION (r149928): CanvasStyle::operator= leaks everything

https://bugs.webkit.org/show_bug.cgi?id=121591

Reviewed by Andreas Kling.

* html/canvas/CanvasStyle.cpp: (WebCore::CanvasStyle::operator=): Don't leak.

Change-Id: I422e07ff97b04c3cd57fdfc437ae12b56ddfedf7
git-svn-id: http://svn.webkit.org/repository/webkit/trunk@156099 268f45cc-cd09-0410-ab3c-d52691b4dbfc
Reviewed-by: Jocelyn Turcotte <jocelyn.turcotte@digia.com>