remove template c++11'ism in Heap.h

Followup to CL 486193002 to remove default arguments from template types,
which is only allowed in c++11 and later (and we haven't officially switched
to c++11 yet afaik).

Review URL: https://codereview.chromium.org/559733004

git-svn-id: svn://svn.chromium.org/blink/trunk@181719 bbb929c8-8fbe-4397-9dbb-9b2b20218538