We'd like to completely replace rtc::scoped_ptr with std::unique_ptr. This is a first trial CL to see if using unique_ptr causes any problems. (As a side effect of removing the scoped_ptr.h include in buffer.h, I had to fix broken includes in no less than three files.) BUG=webrtc:5520 Review URL: https://codereview.webrtc.org/1687833006 Cr-Commit-Position: refs/heads/master@{#11588}