ozone: Fix chromium-style warnings in rendering_helper.cc

ozone: Fix chromium-style warnings in rendering_helper.cc

This fixes the linux_chromium_chromeos_ozone_rel_ng bot:

../../content/common/gpu/media/rendering_helper.cc:105:31: error: [chromium-style] Overriding method must be marked with 'override' or 'final'.
  virtual ~StubOzoneDelegate() {}
                              ^
                               override

TBR=posciak (red build)
NOTRY=true
BUG=none
TEST=linux_chromium_chromeos_ozone_rel_ng trybot

Committed: https://crrev.com/2334c429f4dbe958167ca8e5ab545efb11610951
Cr-Commit-Position: refs/heads/master@{#313763}

[spang, 2015-01-29 19:28:42]

spang@chromium.org changed reviewers:
+ posciak@chromium.org

[spang, 2015-01-29 19:48:20]

The CQ bit was checked by spang@chromium.org

[commit-bot: I haz the power, 2015-01-29 19:49:35]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/875573004/1

[commit-bot: I haz the power, 2015-01-29 19:50:45]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-01-29 19:51:28]

Patchset 1 (id:??) landed as
https://crrev.com/2334c429f4dbe958167ca8e5ab545efb11610951
Cr-Commit-Position: refs/heads/master@{#313763}

[Pawel Osciak, 2015-01-30 04:18:48]

lgtm thanks