Build fix after Blink rev 159806

In the process of rev 159806, an accessor implementation moved to a
different place that caused StyleBuilderCustom.h to have a link error.
This patch addes the #include to fix the build error.

TBR=esprehn@chromium.org, jchaffraix@chromium.org
NOTRY=true

Review URL: https://codereview.chromium.org/26699004

git-svn-id: svn://svn.chromium.org/blink/trunk@159808 bbb929c8-8fbe-4397-9dbb-9b2b20218538