We ported hashmap.h into libsampler as a workaround before, so the main focus of
this patch is to reduce code duplication. This patch moves the hashmap into
src/base as well as creates DefaultAllocationPolicy using malloc and free.

BUG=v8:5050
LOG=n

Review-Url: https://codereview.chromium.org/2010243003
Cr-Commit-Position: refs/heads/master@{#36873}