Extending v8::GetHeapStatistics to return total available size.

For dumping memory statistics about v8 heap, we need the total
available size of the heap. This CL adds extra field in HeapStatistics
to return the available size.

BUG=476013
LOG=Y

Review URL: https://codereview.chromium.org/1104123002

Cr-Commit-Position: refs/heads/master@{#28100}

include/v8.h

@@ -4776,6 +4776,7 @@ class V8_EXPORT HeapStatistics {
   size_t total_heap_size() { return total_heap_size_; }
   size_t total_heap_size_executable() { return total_heap_size_executable_; }
   size_t total_physical_size() { return total_physical_size_; }
+  size_t total_available_size() { return total_available_size_; }
   size_t used_heap_size() { return used_heap_size_; }
   size_t heap_size_limit() { return heap_size_limit_; }
 
@@ -4783,6 +4784,7 @@ class V8_EXPORT HeapStatistics {
   size_t total_heap_size_;
   size_t total_heap_size_executable_;
   size_t total_physical_size_;
+  size_t total_available_size_;
   size_t used_heap_size_;
   size_t heap_size_limit_;
 

src/api.cc

@@ -6871,6 +6871,7 @@ void Isolate::GetHeapStatistics(HeapStatistics* heap_statistics) {
   heap_statistics->total_heap_size_executable_ =
       heap->CommittedMemoryExecutable();
   heap_statistics->total_physical_size_ = heap->CommittedPhysicalMemory();
+  heap_statistics->total_available_size_ = heap->Available();
   heap_statistics->used_heap_size_ = heap->SizeOfObjects();
   heap_statistics->heap_size_limit_ = heap->MaxReserved();
 }