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

webrtc/BUILD.gn

Index: webrtc/BUILD.gn
diff --git a/webrtc/BUILD.gn b/webrtc/BUILD.gn
index 200d2e08cd726dead844a6e47cbfe936dbf1b486..287d224d435500400274f3276f77dc1f7db77abc 100644
--- a/webrtc/BUILD.gn
+++ b/webrtc/BUILD.gn
@@ -18,6 +18,8 @@ import("//third_party/protobuf/proto_library.gni")
 # target_defaults and direct_dependent_settings.
 config("common_inherited_config") {
   defines = []
+  cflags = []
+  ldflags = []
   if (build_with_mozilla) {
     defines += [ "WEBRTC_MOZILLA_BUILD" ]
   }
@@ -77,6 +79,12 @@ config("common_inherited_config") {
     defines += [ "CHROMEOS" ]
   }
 
+  if (rtc_sanitize_coverage != "") {
+    assert(is_clang, "sanitizer coverage requires clang")
+    cflags += [ "-fsanitize-coverage=${rtc_sanitize_coverage}" ]
+    ldflags += [ "-fsanitize-coverage=${rtc_sanitize_coverage}" ]
+  }
+
   # TODO(GYP): Support these in GN.
   # if (is_bsd) {
   #   defines += [ "BSD" ]