Oilpan: Fix CSSValue leak from ImmutableStylePropertySet.

Before this patch, all CSSValues were ref()-ed also on ENABLE(OILPAN) build, without deref().
ImmutableStylePropertySet has a trace implementation so it should not be necessary on ENABLE(OILPAN) build.

The provided test uses svg image to observe SVGDocument increase indirectly held from CSSImageValue.

TEST=fast/css/background-image-leak.html
BUG=None

Review URL: https://codereview.chromium.org/228303003

git-svn-id: svn://svn.chromium.org/blink/trunk@171049 bbb929c8-8fbe-4397-9dbb-9b2b20218538