Make HeapObjectHeader and FinalizedHeapObjectHeader visible in the shared object for newer GCC.
Make HeapObjectHeader and FinalizedHeapObjectHeader visible in the shared object for newer GCC.

When building with newer GCC (4.8) and component=shared_library some classes are
not visible in the created blink_heap shared object.

R=ager@chromium.org
BUG=349932

Review URL: https://codereview.chromium.org/181323005

git-svn-id: bf5cd6ccde378db821296732a091cfbcf5285fbd@168818 bbb929c8-8fbe-4397-9dbb-9b2b20218538

primiano/blink-gitcs,primiano/blink-gitcs,primiano/blink-gitcs,primiano/blink-gitcs,primiano/blink-gitcs,primiano/blink-gitcs,primiano/blink-gitcs,primiano/blink-gitcs,primiano/blink-gitcs