Use relative paths for api/p2p fuzzers.

BUG=
R=aizatsky@chromium.org
TBR=katrielc@webrtc.org

Review URL: https://codereview.webrtc.org/2053093002 .

Cr-Original-Commit-Position: refs/heads/master@{#13095}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: 555cfe9e6e093b01142512de09c8f656308cd043