Gotchas:
If OnMetadataReady never gets called, nothing will ever happen.
When writing tests, AwaitQuiescence doesn't seem to do anything.  Use something like this: https://cs.chromium.org/chromium/src/chrome/browser/sync/test/integration/dictionary_helper.h?l=54&gs=cpp%253Aclass-DictionaryMatchChecker%2540chromium%252F..%252F..%252Fchrome%252Fbrowser%252Fsync%252Ftest%252Fintegration%252Fdictionary_helper.h%257Cdef&gsn=DictionaryMatchChecker&ct=xref_usages to wait for synchronization.

 Issue 666086  has been merged into this issue.

From 666086: "We should have a file (//docs/sync/uss.md probably) that describes a good skeleton ModelTypeSyncBridge and other details about how to implement a USS model type. This should be an evolution of the existing USS design doc into a living document that we keep updated in the repo."

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/119f6c9a974e33480723f429d42e3b787c59c0b5

commit 119f6c9a974e33480723f429d42e3b787c59c0b5
Author: maxbogue <maxbogue@chromium.org>
Date: Wed Feb 01 21:28:46 2017

[Sync] Add the Model API document.

This document contains organic, artisanal links of the highest quality,
hand-crafted for maximum robustness in the face of code change.

Note that you can patch this CL and use
./tools/md_browser/md_browser.py to see a rendered version, though the
styling isn't quite the same as Gitiles.

BUG= 658298 

Review-Url: https://codereview.chromium.org/2669593004
Cr-Commit-Position: refs/heads/master@{#447613}

[add] https://crrev.com/119f6c9a974e33480723f429d42e3b787c59c0b5/docs/sync/model_api.md