Add minimal LLVM sanity coverage (sancov) reporting for unittests.

webrtc/build/webrtc.gni

Index: webrtc/build/webrtc.gni
diff --git a/webrtc/build/webrtc.gni b/webrtc/build/webrtc.gni
index 6370f0e8f66a09e2923433aeb770694b7b400970..30e505d1d796a7852f3d9d9115d7c6018afa0b95 100644
--- a/webrtc/build/webrtc.gni
+++ b/webrtc/build/webrtc.gni
@@ -64,6 +64,13 @@ declare_args() {
 
   rtc_restrict_logging = true
 
+  # Set to "func", "block", "edge" for coverage generation.
+  # At unit test runtime set UBSAN_OPTIONS="coverage=1".
+  # It is recommend to set include_examples=0.
+  # Use llvm's sancov -html-report for human readable reports.
+  # See http://clang.llvm.org/docs/SanitizerCoverage.html .
+  rtc_sanitize_coverage = ""
+
   # Enable libevent task queues on platforms that support it.
   if (is_win || is_mac || is_ios || is_nacl) {
     rtc_enable_libevent = false