buffers.h now only contains functions for creating timestamps. This
change also removes 'extern' from the functions in
timestamp_constants.h (because it doesn't mean anything on a
function definition), stops including it in files that don't use
it, and adds includes to files that use it without including it.

BUG=528114
TEST=It compiles

Review URL: https://codereview.chromium.org/1321473004

Cr-Commit-Position: refs/heads/master@{#347491}