Delay completion of getUserMedia in UserMediaClient

This fixes a problem when the RenderFrame is destroyed within the context of a getusermedia callback.
BUG=472617, 474370
TBR=tommi@chromium.org

Committed: https://crrev.com/fa7c6fd4aa278dab68c5987b45353e064ce8e0ad
Cr-Commit-Position: refs/heads/master@{#324434}

[perkj_chrome, 2015-04-09 08:52:57]

Patchset #1 (id:1) has been deleted

[perkj_chrome, 2015-04-09 12:54:51]

perkj@chromium.org changed reviewers:
+ magjed@chromium.org, tommi@chromium.org

[perkj_chrome, 2015-04-09 12:54:52]

Please?

[magjed_chromium, 2015-04-09 14:23:03]

lgtm

[perkj_chrome, 2015-04-09 14:29:10]

The CQ bit was checked by perkj@chromium.org

[commit-bot: I haz the power, 2015-04-09 14:29:22]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1075833002/20001

[commit-bot: I haz the power, 2015-04-09 14:33:28]

Committed patchset #1 (id:20001)

[commit-bot: I haz the power, 2015-04-09 14:34:17]

Patchset 1 (id:??) landed as
https://crrev.com/fa7c6fd4aa278dab68c5987b45353e064ce8e0ad
Cr-Commit-Position: refs/heads/master@{#324434}