Require V8 to be explicitly initialized before an Isolate is created

We also initialize the Isolate on creation.

This should allow for getting rid of the last remaining default isolate
traces. Also, it'll speed up several isolate related operations that no
longer require locks.

Embedders that relied on v8::Isolate to return an uninitialized Isolate
(so they can set ResourceConstraints for example, or set flags that
modify the way the isolate is created) should either do the setup before
creating the isolate, or use the recently added CreateParams to pass e.g.
ResourceConstraints.

BUG=none
LOG=y
R=svenpanne@chromium.org

Review URL: https://codereview.chromium.org/469783002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@24052 ce2b1a6d-e550-0410-aec6-3dcde31c8c00