Blink GC plugin: Check polymorphic requirements on all GC-derived classes.

Blink GC plugin: Check polymorphic requirements on all GC-derived classes.

Prior to this change we only checked the polymorphic requirements on classes
that declared a trace method. Also, this fixes a typo in condition (2) where we
checked on the derived class instead of the left-most class.

R=ager@chromium.org
BUG=

Committed: https://crrev.com/f691be3b1bbbfb491e9afd283fe100acda894147
Cr-Commit-Position: refs/heads/master@{#294360}

[zerny-chromium, 2014-09-10 13:22:45]

Fixes unreported error issue as found in:
https://codereview.chromium.org/562573002/

Compiling with this fix does not find any issues apart from the known
MIDI{Input,Output}Map issue which are correctly reported.

[Mads Ager (chromium), 2014-09-10 14:39:45]

LGTM

[wibling-chromium, 2014-09-10 15:58:16]

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

[wibling-chromium, 2014-09-10 15:58:16]

lgtm

[haraken, 2014-09-10 15:59:31]

LGTM

[zerny-chromium, 2014-09-11 07:55:50]

The CQ bit was checked by zerny@chromium.org

[commit-bot: I haz the power, 2014-09-11 08:01:32]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patchset/564453002/20001

[commit-bot: I haz the power, 2014-09-11 09:01:43]

Committed patchset #2 (id:20001) as 76819f6eb89d7663bf9e91e16dfcf55430b8802f

[commit-bot: I haz the power, 2014-09-11 09:03:51]

Patchset 2 (id:??) landed as
https://crrev.com/f691be3b1bbbfb491e9afd283fe100acda894147
Cr-Commit-Position: refs/heads/master@{#294360}