Rename media/base/buffers.h to media/base/timestamp_constants.h

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}

diff --git a/media/filters/decrypting_audio_decoder_unittest.cc b/media/filters/decrypting_audio_decoder_unittest.cc
index 59d7a3e..d6ef2b9 100644
--- a/media/filters/decrypting_audio_decoder_unittest.cc
+++ b/media/filters/decrypting_audio_decoder_unittest.cc

@@ -9,12 +9,12 @@
 #include "base/callback_helpers.h"
 #include "base/message_loop/message_loop.h"
 #include "media/base/audio_buffer.h"
-#include "media/base/buffers.h"
 #include "media/base/decoder_buffer.h"
 #include "media/base/decrypt_config.h"
 #include "media/base/gmock_callback_support.h"
 #include "media/base/mock_filters.h"
 #include "media/base/test_helpers.h"
+#include "media/base/timestamp_constants.h"
 #include "media/filters/decrypting_audio_decoder.h"
 #include "testing/gmock/include/gmock/gmock.h"