This helps a lot on Android devices where the user threads can be scheduled with low priority when the app is in the background, causing spurious significantly delayed before a packet can be read from the socket. With this patch the timestamp is taken by the kernel when the packet actually arrives. R=juberti@chromium.org TBR=juberti@webrtc.org BUG=webrtc:5773 Review URL: https://codereview.webrtc.org/1944683002 . Cr-Commit-Position: refs/heads/master@{#12850}