Fix build by guarding tracing of m_holder in ScriptPromiseProperty.

ScriptPromisePropertyBase is RefCounted in the non-oilpan build
so this object is never traced in the non-oilpan build and the
ifdef is safe.

TBR=haraken@chromium.org,sigbjornf@opera.com
NOTRY=true
NOTREECHECKS=true

Review URL: https://codereview.chromium.org/396903002

git-svn-id: svn://svn.chromium.org/blink/trunk@178252 bbb929c8-8fbe-4397-9dbb-9b2b20218538