Make DOMWrapperWorlds easier to use

Make DOMWrapperWorlds easier to use

This patch adds a DOMWrapperWorld::current() static method that
returns either the DOMWrapperWorld for the current V8 context or
null if in a Worker.

Recently-transitioned callsites that previously used WorkerContextHandle
have been updated to call DOMWrapperWorld::current() instead, and it
will be used in upcoming WorldContextHandle transitions.

This lets us remove a FIXME in toV8Context() since all non-Worker
callers now pass in a non-null DOMWrapperWorld (rather than the
old behavior of passing null as a sentinel for "main world").

R=abarth
BUG=230460
Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=148501

[adamk, 2013-04-16 20:33:01]

Note that we discussed offline whether workers should have their own
DOMWrapperWorld, rather than using a null pointer. I was going to do this until
I realized it'd require either one world object per worker or a thread-safe
DOMWrapperWorld. Given that there's no other use-case right not for a
non-main-thread DOMWrapperWorld, I'm leaving it at the status quo for now.

[abarth-chromium, 2013-04-16 20:56:51]

Looks great!  LGTM

[adamk, 2013-04-16 22:07:28]

Committed patchset #1 manually as r148501 (presubmit successful).