Revert of Move Extension ScriptContext creation into ScriptContextSet.

Revert of Move Extension ScriptContext creation into ScriptContextSet. (patchset #3 id:40001 of https://codereview.chromium.org/995283004/)

Reason for revert:
Broke ScriptContextTest.GinRunnerLifetime.

Original issue's description:
> Move Extension ScriptContext creation into ScriptContextSet.
>
> Previously extensions::Dispatcher was responsible for creating ScriptContexts
> then Adding them to its ScriptContextSet. Now, ScriptContextSet creates them
> itself. This prepares the Extension system for later changing to lazily or
> asynchronously creating ScriptContexts.
>
> BUG=466373
> R=rockot@chromium.org
>
> Committed: https://crrev.com/75605c7fca774b95984ba67f70ba86bb5750c0f6
> Cr-Commit-Position: refs/heads/master@{#320602}

TBR=rockot@chromium.org
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=466373
Committed: https://crrev.com/8ee8ee9e645596d0eabda36b30e3b99c24920940
Cr-Commit-Position: refs/heads/master@{#320626}

[not at google - send to devlin, 2015-03-14 00:50:56]

Created Revert of Move Extension ScriptContext creation into ScriptContextSet.

[commit-bot: I haz the power, 2015-03-14 00:51:23]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1010473002/1

[commit-bot: I haz the power, 2015-03-14 00:52:36]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-03-14 00:53:51]

Patchset 1 (id:??) landed as
https://crrev.com/8ee8ee9e645596d0eabda36b30e3b99c24920940
Cr-Commit-Position: refs/heads/master@{#320626}