Introduce TaskLoop::BindToCurrentThread and TaskLoop and TaskRunner handles

[domfarolino]

Closes https://github.com/domfarolino/browser/issues/15.

This PR introduces:

• A thread-local (or thread-global depending on how you see it) pointer to the current thread's base::TaskRunner
• A thread-local pointer to the current thread's base::TaskLoop

... as well as a:

• A process-global pointer to the process's UI thread's TaskLoop
• A process-global pointer to the process's IO thread's TaskLoop

These pointers are set by TaskLoop::BindToCurrentThread(), which is called by TaskLoop::Create. We also expose TaskLoop::CreateUnbound so that you can create TaskLoops that are not bound upon construction. This is important for base::Thread, which is currently the only non-test-only consumer of TaskLoop. base::Thread creates an unbound TaskLoop in base::Thread::Start, and then spins up a new physical thread. When the new thread is up and running, we then bind the TaskLoop to the current thread.

(After this PR we'll need to add base::Thread tests, since we are seriously lacking test support there)

[domfarolino]

FYI @pmusgrave @musgravejw feel free to take a look if you want. This PR is an exciting one in that it lets you access cross-thread TaskLoops without having an explicit reference a base::Thread or a base::TaskLoop that is bound to another thread. I'll probably land this soon.