[heap] Hide MemoryReducer inside the heap component.

[heap] Hide MemoryReducer inside the heap component.

This prevents leakage of the memory-reducer.h declarations inside of the
heap and prevents it from being exposed to the world. Protects private
state from being inadvertently mocked with.

R=mlippautz@chromium.org

Review URL: https://codereview.chromium.org/1288913003

Cr-Commit-Position: refs/heads/master@{#30277}