Fix exception on navigation while "Recoding object allocations"

Current profile may have been removed by the time didTakeHeapSnapshot is invoked.

BUG=None
R=alph@chromium.org

Review URL: https://codereview.chromium.org/167003006

git-svn-id: svn://svn.chromium.org/blink/trunk@167336 bbb929c8-8fbe-4397-9dbb-9b2b20218538