[Sync] Handle invalid specifics field numbers gracefully

Change GetModelTypeFromSpecificsFieldNumber() to not NOTREACHED() on an
unknown field number.  Instead, have callers compare the return value
to UNSPECIFIED and handle that case.

BUG=165171


Review URL: https://chromiumcodereview.appspot.com/11490018

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@172232 0039d316-1c4b-4281-b951-d872f2087c98
5 files changed