When the main thread terminates, it forcibly terminates all Worker threads.
When this happens, the threads objects were only half-created; they had a
JavaScript Worker object, but not a C++ worker object.

This CL fixes that bug, as well as some other fixes:
* Signatures on Worker methods
* Use SetAlignedPointerFromInternalField instead of using an External.
* Remove state_ from Worker. Simplify to atomic bool running_.

BUG=chromium:511880
R=jarin@chromium.org
LOG=n

Review URL: https://codereview.chromium.org/1255563002

Cr-Commit-Position: refs/heads/master@{#29911}