Oilpan: fix MSVC compilation after r180556.

Have the "same-size" class inherit from the same classes as CSSRule, so
as to have the MSVC-specific alignment constraints of the parent
ScriptWrappableBase class apply in equal measure.

TBR=oilpan-reviews@chromium.org,yukishiino@chromium.org
BUG=235436
NOTRY=true

Review URL: https://codereview.chromium.org/487923002

git-svn-id: svn://svn.chromium.org/blink/trunk@180601 bbb929c8-8fbe-4397-9dbb-9b2b20218538