Add rtc_event_log_unittest_helper.h to relevant BUILD.gn

BUG=None

Review-Url: https://codereview.webrtc.org/3002813002
Cr-Original-Commit-Position: refs/heads/master@{#19389}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: b5c319ad8bc86204825ddeb88dcc482348686ff5
1 file changed