Move a bunch of GC related files to heap/ subdirectory

src/heap/gc-tracer.h

 // Copyright 2014 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-#ifndef V8_GC_TRACER_H_
-#define V8_GC_TRACER_H_
+#ifndef V8_HEAP_GC_TRACER_H_
+#define V8_HEAP_GC_TRACER_H_
 
 namespace v8 {
 namespace internal {
 
 // A simple ring buffer class with maximum size known at compile time.
 // The class only implements the functionality required in GCTracer.
 template <typename T, size_t MAX_SIZE>
 class RingBuffer {
  public:
   class const_iterator {
@@ skipping 315 matching lines @@
   // TODO(hpayer): This timer right now just holds the sweeping time
   // of the initial atomic sweeping pause. Make sure that it accumulates
   // all sweeping operations performed on the main thread.
   double cumulative_sweeping_duration_;
 
   DISALLOW_COPY_AND_ASSIGN(GCTracer);
 };
 }
 }  // namespace v8::internal
 
-#endif  // V8_GC_TRACER_H_
+#endif  // V8_HEAP_GC_TRACER_H_