Enable GN check for webrtc/{ortc,p2p}

Introduce new target //webrtc/p2p:rtc_p2p_test_utils to host
test-related utilities.
Previously uncovered header "base/fakecandidatepair.h" is now also in a target.

BUG=webrtc:6828

Review-Url: https://codereview.webrtc.org/2714263004
Cr-Original-Commit-Position: refs/heads/master@{#17036}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: c9515b6ce642875c4994deb25a5bb0132a958848
2 files changed