Remove remaining mentions of gflags

BUG=webrtc:7644

Review-Url: https://codereview.webrtc.org/3011413002
Cr-Commit-Position: refs/heads/master@{#19950}
diff --git a/.gn b/.gn
index 882d220..c3b2677 100644
--- a/.gn
+++ b/.gn
@@ -67,4 +67,8 @@
 
   # WebRTC relies on Chromium's Android test infrastructure.
   use_cxx11_on_android = false
+
+  # WebRTC does not provide the gflags dependency. Because libyuv uses it only
+  # for unittests, it can be disabled (see third_party/libyuv/BUILD.gn)
+  libyuv_use_gflags = false
 }