Exit mksnapshot cleanly, making LSAN happy.

R=mstarzinger@chromium.org

Review URL: https://codereview.chromium.org/137823009

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@18904 ce2b1a6d-e550-0410-aec6-3dcde31c8c00

src/mksnapshot.cc

@@ -399,5 +399,8 @@ int main(int argc, char** argv) {
       ser.CurrentAllocationAddress(i::MAP_SPACE),
       ser.CurrentAllocationAddress(i::CELL_SPACE),
       ser.CurrentAllocationAddress(i::PROPERTY_CELL_SPACE));
+  isolate->Exit();
+  isolate->Dispose();
+  V8::Dispose();
   return 0;
 }