Fix crash in webmidi.

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
Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=181718

[yhirano, 2014-09-10 07:57:26]

Thanks, lgtm

[haraken, 2014-09-10 07:59:41]

LGTM, nice detective work!

[Mads Ager (chromium), 2014-09-10 08:24:39]

The CQ bit was checked by ager@chromium.org

[commit-bot: I haz the power, 2014-09-10 08:25:33]

CQ is trying da patch. Follow status at
 https://chromium-status.appspot.com/cq/ager@chromium.org/562573002/1

[commit-bot: I haz the power, 2014-09-10 09:23:48]

Committed patchset #1 (id:1) as 181718

[wibling-chromium, 2014-09-10 09:31:21]

wibling@chromium.org changed reviewers:
+ wibling@chromium.org

[wibling-chromium, 2014-09-10 09:31:22]

lgtm

[zerny-chromium, 2014-09-10 10:55:07]

lgtm. This was not caught by the plugin because we only checked the polymorphic
requirements on classes defining a trace method, which MIDI{Input,Output}Map
don't. I'll upload a plugin fix asap.

[zerny-chromium, 2014-09-10 13:19:46]

On 2014/09/10 10:55:07, zerny-chromium wrote:
> lgtm. This was not caught by the plugin because we only checked the
polymorphic
> requirements on classes defining a trace method, which MIDI{Input,Output}Map
> don't. I'll upload a plugin fix asap.

Plugin fix uploaded at: https://codereview.chromium.org/564453002/