Fix crash in webmidi.

If a garbage collected class is polymorphic (has virtual methods) it
needs to have a vtable from its left-most base class. Otherwise
pointers to the polymorphic subclasses are not guaranteed to point
to the beginning of the object.

This should have been caught by the gc plugin. We will investigate
that separately.

R=oilpan-reviews@chromium.org, sigbjornf@chromium.org, yhirano@chromium.org, zerny@chromium.org
BUG=411742

Review URL: https://codereview.chromium.org/562573002

git-svn-id: svn://svn.chromium.org/blink/trunk@181718 bbb929c8-8fbe-4397-9dbb-9b2b20218538