Adding missing V8_EXPORT flag in SpaceStatistics class in v8.h

This class was added in crrev.com/1058253003 and missed V8_EXPORT in
definition.

BUG=466141, 476013
LOG=Y

Review URL: https://codereview.chromium.org/1090333002

Cr-Commit-Position: refs/heads/master@{#27926}

include/v8.h

@@ -4901,7 +4901,7 @@ class V8_EXPORT HeapStatistics {
 };
 
 
-class HeapSpaceStatistics {
+class V8_EXPORT HeapSpaceStatistics {
  public:
   HeapSpaceStatistics();
   const char* space_name() { return space_name_; }