[cpu-profiler] Use std::unordered_map for hashmaps.

There doesn't seem to be any reason to use our custom hashmap here,
which has a more complicated interface.

Change-Id: Ib08c2e400a3cb402a5984b925034aac29750c2ec
Reviewed-on: https://chromium-review.googlesource.com/1019445
Reviewed-by: Yang Guo <yangguo@chromium.org>
Commit-Queue: Peter Marshall <petermarshall@chromium.org>
Cr-Commit-Position: refs/heads/master@{#52724}