Make MatchFun a template parameter in TemplateHashMapImpl, moving the
PointersMatch function down to an implementation which extends
TemplateHashMapImpl to void* key and value (i.e. the same as the current
HashMap and ZoneHashMap typedefs).

This will allow other instantiations of TemplateHashMapImpl, with
different MatchFun values, e.g. std::equal_to, to have their key
equality test inlined, rather than calling a function pointer,

Review-Url: https://codereview.chromium.org/2354593002
Cr-Commit-Position: refs/heads/master@{#39868}