[heap] Hide GCTracer inside the heap component.

[heap] Hide GCTracer inside the heap component.

This prevents leakage of the gc-tracer.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/1294763004

Cr-Commit-Position: refs/heads/master@{#30276}