Split TemplateHashMapImpl::Lookup into two methods

This avoids both a mysterious boolean argument ("insert") and lets
non-mutating lookups skip passing an allocator (in one such case,
we were passing a scary-looking ZoneAllocationPolicy(NULL)!).

Review URL: https://codereview.chromium.org/1074943002

Cr-Commit-Position: refs/heads/master@{#27799}