Cleanup after EmbedderHeapTracer api updates

In order to update v8 api and still be able to build chromium, we had to keep
code in suboptimal state. Now when chromium caught up, we can perform cleanup :)

LOG=no
BUG=468240

Review-Url: https://codereview.chromium.org/2057613002
Cr-Commit-Position: refs/heads/master@{#36933}

include/v8.h

@@ -5447,15 +5447,8 @@ class V8_EXPORT EmbedderHeapTracer {
    * Embedder is expected to store them in it's marking deque and trace
    * reachable wrappers from them when asked by AdvanceTracing method.
    */
-  // TODO(hlopko): Make pure virtual after migration
   virtual void RegisterV8References(
-      const std::vector<std::pair<void*, void*> >& internal_fields) {}
-  /**
-   * **Deprecated**
-   */
-  // TODO(hlopko): Remove after migration
-  virtual void TraceWrappersFrom(
-      const std::vector<std::pair<void*, void*> >& internal_fields) {}
+      const std::vector<std::pair<void*, void*> >& internal_fields) = 0;
   /**
    * V8 will call this method at the beginning of the gc cycle.
    */
@@ -5468,11 +5461,8 @@ class V8_EXPORT EmbedderHeapTracer {
    *
    * Returns true if there is still work to do.
    */
-  // TODO(hlopko): Make pure virtual after migration
   virtual bool AdvanceTracing(double deadline_in_ms,
-                              AdvanceTracingActions actions) {
-    return false;
-  }
+                              AdvanceTracingActions actions) = 0;
   /**
    * V8 will call this method at the end of the gc cycle. Allocation is *not*
    * allowed in the TraceEpilogue.