Sort #includes that got unsorted when gmock.h and gtest.h moved to webrtc/test/

gmock.h and gtest.h were moved (or rather, got wrappers so that we
could put some icky compatibility hacks in one place instead of 500)
in this CL: https://codereview.webrtc.org/2358993004/

NOPRESUBMIT=true
BUG=webrtc:6398

Review-Url: https://codereview.webrtc.org/2381013002
Cr-Commit-Position: refs/heads/master@{#14464}

webrtc/modules/video_coding/video_packet_buffer_unittest.cc

@@ -13,11 +13,11 @@
 #include <set>
 #include <utility>
 
-#include "webrtc/test/gtest.h"
 #include "webrtc/base/random.h"
 #include "webrtc/modules/video_coding/frame_object.h"
 #include "webrtc/modules/video_coding/packet_buffer.h"
 #include "webrtc/system_wrappers/include/clock.h"
+#include "webrtc/test/gtest.h"
 
 namespace webrtc {
 namespace video_coding {