[heap] Avoid overzealous inclusion of heap internal headers.

[heap] Avoid overzealous inclusion of heap internal headers.

This is a first step towards constraining down the heap interface to
just the heap.h file. Note that many includes still leak through that
file to the global "src" directory, but there now is a single place
controlling which declarations leak that way. Especially inclusion of
inline header files within "heap" has been limited drastically.

R=hpayer@chromium.org,mlippautz@chromium.org

Review URL: https://codereview.chromium.org/1281233003

Cr-Commit-Position: refs/heads/master@{#30092}