This cl introduces reporter to the embedder heap tracer. Heap tracer uses
reporter to report reachable objects from embedder heap. This refactoring is a
step towards being able to take heap snapshots with wrapper tracing.

LOG=no
BUG=468240

Review-Url: https://codereview.chromium.org/2371733002
Cr-Commit-Position: refs/heads/master@{#39824}