extensions: Speculative fix for crash on IO thread.

extensions: Speculative fix for crash on IO thread.

Make ExtensionFunctionDispatcher::DispatchOnIOThread()
return early if the extension that was passed to it doesn't
exist anymore.

BUG=406062
Committed: https://crrev.com/5122f4eb8fa975f8e7037e5c27075e0acb3c0890
Cr-Commit-Position: refs/heads/master@{#291618}

[not at google - send to devlin, 2014-08-23 19:27:06]

lgtm

[Daniel Erat, 2014-08-23 20:10:19]

The CQ bit was checked by derat@chromium.org

[commit-bot: I haz the power, 2014-08-23 20:11:18]

CQ is trying da patch. Follow status at
 https://chromium-status.appspot.com/cq/derat@chromium.org/489033003/1

[commit-bot: I haz the power, 2014-08-23 21:29:10]

FYI, CQ is re-trying this CL (attempt #1).
The failing builders are:
  win8_chromium_rel on tryserver.chromium.win
(http://build.chromium.org/p/tryserver.chromium.win/builders/win8_chromium_rel...)
  win_chromium_x64_rel_swarming on tryserver.chromium.win
(http://build.chromium.org/p/tryserver.chromium.win/builders/win_chromium_x64_...)

[commit-bot: I haz the power, 2014-08-24 04:10:44]

Committed patchset #1 (1) as b3fca8f3f90ffec11c6cf9d923f142ae6fa8f165

[commit-bot: I haz the power, 2014-09-10 02:32:20]

Patchset 1 (id:??) landed as
https://crrev.com/5122f4eb8fa975f8e7037e5c27075e0acb3c0890
Cr-Commit-Position: refs/heads/master@{#291618}