[heap] Force inlining of AllocatedSinceLastGC

Speculatively forcining inlining as not inlining potentially regresses
performance.

BUG=chromium:605524
LOG=N

Review-Url: https://codereview.chromium.org/1924033003
Cr-Commit-Position: refs/heads/master@{#35869}

src/heap/heap.h

@@ -1256,8 +1256,8 @@ class Heap {
     return static_cast<intptr_t>(total);
   }
 
-  inline void UpdateNewSpaceAllocationCounter();
-  inline size_t NewSpaceAllocationCounter();
+  V8_INLINE void UpdateNewSpaceAllocationCounter();
+  V8_INLINE size_t NewSpaceAllocationCounter();
 
   // This should be used only for testing.
   void set_new_space_allocation_counter(size_t new_value) {

src/heap/spaces.h

@@ -2601,7 +2601,7 @@ class NewSpace : public Space {
   // Return the available bytes without growing.
   intptr_t Available() override { return Capacity() - Size(); }
 
-  inline size_t AllocatedSinceLastGC();
+  V8_INLINE size_t AllocatedSinceLastGC();
 
   bool ReplaceWithEmptyPage(Page* page) {
     // This method is called after flipping the semispace.