Store the RenderProcessHostFactory in a global variable

Store the RenderProcessHostFactory in a global variable
instead of a member variable within each SiteInstance.

The local variable in one SiteInstance was documented to be passed on
to other SiteInstances spawned from it, but that never actually
worked.  Even fixing that doesn't serve the use case of allowing
different RenderProcessHostFactories for different trees of
SiteInstances because of the many calls to SiteInstance::CreateForURL
inside RenderViewHostManager.

Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=204409

[Charlie Reis, 2013-06-05 18:43:09]

LGTM

[commit-bot: I haz the power, 2013-06-05 19:32:57]

CQ is trying da patch. Follow status at
https://chromium-status.appspot.com/cq/jyasskin@chromium.org/15799009/1

[commit-bot: I haz the power, 2013-06-06 03:08:35]

Commit queue rejected this change because the description was changed
between the time the change entered the commit queue and the time it
was ready to commit. You can safely check the commit box again.

[commit-bot: I haz the power, 2013-06-06 03:15:29]

CQ is trying da patch. Follow status at
https://chromium-status.appspot.com/cq/jyasskin@chromium.org/15799009/1

[commit-bot: I haz the power, 2013-06-06 03:48:52]

Change committed as 204409