Move gmock_callback_support.h to //base/test

Also update all references to it, and get rid of some other (partial) copies
of the file as well.

Tbr: ccameron@chromium.org
Bug: none
Change-Id: I22f5e59d9d5206a71ffcccb7b9c2a6447bb8dbd5
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1639659
Commit-Queue: Marijn Kruisselbrink <mek@chromium.org>
Reviewed-by: Rouslan Solomakhin <rouslan@chromium.org>
Reviewed-by: Xiaohan Wang <xhwang@chromium.org>
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Cr-Commit-Position: refs/heads/master@{#666491}

diff --git a/media/filters/decrypting_audio_decoder_unittest.cc b/media/filters/decrypting_audio_decoder_unittest.cc
index 62a6fd5..e1d52e6 100644
--- a/media/filters/decrypting_audio_decoder_unittest.cc
+++ b/media/filters/decrypting_audio_decoder_unittest.cc

@@ -11,11 +11,11 @@
 #include "base/callback_helpers.h"
 #include "base/run_loop.h"
 #include "base/stl_util.h"
+#include "base/test/gmock_callback_support.h"
 #include "base/test/scoped_task_environment.h"
 #include "media/base/audio_buffer.h"
 #include "media/base/decoder_buffer.h"
 #include "media/base/decrypt_config.h"
-#include "media/base/gmock_callback_support.h"
 #include "media/base/media_util.h"
 #include "media/base/mock_filters.h"
 #include "media/base/test_helpers.h"
@@ -23,6 +23,7 @@
 #include "media/filters/decrypting_audio_decoder.h"
 #include "testing/gmock/include/gmock/gmock.h"
 
+using ::base::test::RunCallback;
 using ::testing::_;
 using ::testing::AtMost;
 using ::testing::Return;