Fixing Heap::Available() to return total of all spaces.

The Heap::Available method adds up the available size from only 4 of
the spaces. This CL fixes the method to return total of all spaces.

BUG=481504
LOG=N

Review URL: https://codereview.chromium.org/1141693003

Cr-Commit-Position: refs/heads/master@{#28566}

src/heap/heap.cc

@@ -229,8 +229,12 @@ void Heap::UpdateMaximumCommitted() {
 intptr_t Heap::Available() {
   if (!HasBeenSetUp()) return 0;
 
-  return new_space_.Available() + old_space_->Available() +
-         code_space_->Available() + map_space_->Available();
+  intptr_t total = 0;
+  AllSpaces spaces(this);
+  for (Space* space = spaces.next(); space != NULL; space = spaces.next()) {
+    total += space->Available();
+  }
+  return total;
 }