Replace gflags usages with rtc_base/flags in all targets based on test_main

BUG=webrtc:7644

Review-Url: https://codereview.webrtc.org/2995363002
Cr-Original-Commit-Position: refs/heads/master@{#19580}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: 9b2f20c6187f29e0c5105ff436b2fc787f924976