Remove base/synchronization/, move src/ to util/

Moving src/ to util/ (and removing src/ from include path) was necessary
so that Semaphore could live in "util/semaphore.h" rather than having a
different name (because <semaphore.h> is where sem_t is on Linux).
Unfortunately this required touching ~everything because build_config.h
needed to be changed to util/build_config.h.

Change-Id: I4c913542c1f1504dca1c82e6ab6d99ef263f5cac
Reviewed-on: https://gn-review.googlesource.com/1680
Commit-Queue: Scott Graham <scottmg@chromium.org>
Reviewed-by: Brett Wilson <brettw@chromium.org>