Avoid creating a second block heap manager in the unittests.

Presently there's 2 BHMs in the BHM unittests, the one created by the runtime and the one created dynamically in the test, this is useless as we can directly use the one from the runtime...

BUG=
R=chrisha@chromium.org

Review URL: https://codereview.appspot.com/141140043