Index script compilation cache over context

Index script compilation cache over context, too,
in preparation for global lexical scope.

R=ulan@chromium.org
BUG=
TEST=

Committed: https://code.google.com/p/v8/source/detail?r=12397

[ulan, 2012-08-22 14:29:49]

Could you please check that with this CL we still get compilation cache hits if
we reload a webpage in Chrome?

[rossberg, 2012-08-24 12:47:04]

On 2012/08/22 14:29:49, ulan wrote:
> Could you please check that with this CL we still get compilation cache hits
if
> we reload a webpage in Chrome?

I checked with Chromium HEAD and a few reloads, and it had almost identical hit
logs before and after the change.

I think this should all work out. There are three main scenarios I see:

1. Reload always operates in the empty environment, i.e. always sees the same
bare native context, such that cache hits aren't be affected.

2. Likewise, any page that does not define something in the global lexical scope
(i.e. not using let, Harmony const, module) shouldn't be affected, because empty
contexts aren't represented.

3. But even a page that _does_ use global lexical scope (i.e. future ES6
applications) still remains cachable as long as all its scripts with lexical
content are executed in a deterministic (i.e. repeatable) order. For script
tags, that always is the case.

Through reflective loads it will probably be possible to defeat all this, but
then you're asking for it. :)

[rossberg, 2012-08-24 12:50:11]

On 2012/08/24 12:47:04, rossberg wrote:
> Through reflective loads it will probably be possible to defeat all this, but
> then you're asking for it. :)

To clarify: reflective loads of ES6 code _with_ lexical content. I.e., nothing
today's web pages contain.

[ulan, 2012-08-28 08:54:07]

LGTM. Can we remove ", too," from the description?