heap: Another round of Thread{State,Heap} API cleanups

- Move PrefinalizerRegistration to top
- More documentation
- Move schedule helpers to private section
- Remove dead code

Bug: 982754
Change-Id: I8eaa76bae7f18cc881b66e65df099806a3234f99
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1745988
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Kentaro Hara <haraken@chromium.org>
Auto-Submit: Michael Lippautz <mlippautz@chromium.org>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Cr-Commit-Position: refs/heads/master@{#685523}

third_party/blink/renderer/platform/heap/heap.h

@@ -182,11 +182,6 @@ class PLATFORM_EXPORT ThreadHeap {
   explicit ThreadHeap(ThreadState*);
   ~ThreadHeap();
 
-  // Returns true for main thread's heap.
-  // TODO(keishi): Per-thread-heap will return false.
-  bool IsMainThreadHeap() { return this == ThreadHeap::MainThreadHeap(); }
-  static ThreadHeap* MainThreadHeap() { return main_thread_heap_; }
-
   template <typename T>
   static inline bool IsHeapObjectAlive(const T* object) {
     static_assert(sizeof(T), "T must be fully defined");