Move implementation of ChildProcessSecurityPolicyImpl to c/b/shared, and wrap in c/b

Move implementation of ChildProcessSecurityPolicyImpl to c/b/shared, and wrap in c/b

I tried to do the refactor all-in-one but it got too complicated. So as
a first step, move the implementation of ChildProcessSecurityPolicyImpl
to c/b/shared as ChildProcessSecurityPolicyHelper, which implements the
same interface and holds all data (but isn't directly an implementation
of ChildProcessSecurityPolicy), and then have
ChildProcessSecurityPolicyImpl forward to it.

The idea for followups would be to pick apart the implementation
hopefully removing parts the loader doesn't need. I'm not really sure
how that'll work yet though.

TODO c/b/shared is intended for code to be shared between browser and
loader, needs a better name before landing.

R=jam
BUG=598073, 624183

[scottmg, 2016-07-01 00:09:51]

The CQ bit was checked by scottmg@chromium.org to run a CQ dry run

[commit-bot: I haz the power, 2016-07-01 00:10:19]

Dry run: CQ is trying da patch. Follow status at
 
https://chromium-cq-status.appspot.com/v2/patch-status/codereview.chromium.or...

[commit-bot: I haz the power, 2016-07-01 01:07:59]

The CQ bit was unchecked by commit-bot@chromium.org

[commit-bot: I haz the power, 2016-07-01 01:07:59]

Dry run: This issue passed the CQ dry run.