[Oilpan]: Check if virtual class objects have been initialized during conservative scanning.

When allocating an object that itself allocates objects as parameters for its constructor we saw that a GC could
occur while the outermost object was not yet initialized. This meant that we could push an invalid trace method on the tracing stack.

Basically C++ will not initialize the object's vtable until after the arguments for the constructor has been evaluated. When
the evaluation of the arguments triggered a GC we risked pushing an object with a NULL trace method on the tracing stack
during conservative stack scanning.
To avoid this issue we instead check, during conservative stack scanning, whether the object we are about to mark is virtual
and if so whether its vtable has been initialized. If not we scan the object conservatively as well.

R=ager@chromium.org, erik.corry@gmail.com, haraken@chromium.org, oilpan-reviews@chromium.org, tkent@chromium.org, vegorov@chromium.org, zerny@chromium.org
BUG=

Review URL: https://codereview.chromium.org/230223007

git-svn-id: svn://svn.chromium.org/blink/trunk@171152 bbb929c8-8fbe-4397-9dbb-9b2b20218538