Don't look for window in IsolatedWorld

This causes crashes in GetTemplate because we are using a non-WebKit v8
context, which doesn't have the template (and allocation has been disabled,
need to find out where this is being disabled).

For now, if we don't have any isolated worlds, then don't bother checking it.
Don't use calling context if it is not WebKit's

DRQS 41162567

The calling context used for activeContext() might be a non-WebKit context, in
which case we treat it like it doesn't exist at all, and we fall back to the
entered context.
Don't assert domWindow in FeatureObserver

DRQS 41162567

FeatureObserver is not very important to us.  It is ok if this is a no-op when
the domWindow is null.
Merge remote-tracking branch 'origin/bugfix/nonWindowContexts/26.0.1410.33' into bugfix/nonWindowContexts/27.0.1453.73