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

webrtc/build/common.gypi

Index: webrtc/build/common.gypi
diff --git a/webrtc/build/common.gypi b/webrtc/build/common.gypi
index 36a2daedf03eee876483d7e1a1cecfeb00200195..434e0a425819d56c63d92ed2bc3b1ebea530b20f 100644
--- a/webrtc/build/common.gypi
+++ b/webrtc/build/common.gypi
@@ -98,6 +98,13 @@
     # Set to 1 to enable code coverage on Linux using the gcov library.
     'coverage%': 0,
 
+    # 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 .
+    'webrtc_sanitize_coverage%': "",

[phoglund, 2016/07/19 19:12:40]

We need to have parity between gn and gyp; please add this variable to
build/webrtc.gni, and the flag fiddling to webrtc/BUILD.gn (in
common_inherited_config). Ensure it works with for instance

gn gen out/Default --args="webrtc_sanitize_coverage=func"
ninja -C out/Default some_target

[johan, 2016/07/21 12:33:22]

Done, gn files use name pattern "rtc_.*" while gyp files use 'webrtc_.*' for
arguments.
gn gen out/Default --args='rtc_sanitize_coverage="func"'

+
     # Remote bitrate estimator logging/plotting.
     'enable_bwe_test_logging%': 0,
 
@@ -404,6 +411,17 @@
         'ldflags': [ '--coverage' ],
         'link_settings': { 'libraries': [ '-lgcov' ] },
       }],
+     ['webrtc_sanitize_coverage!=""', {
+        'cflags': [ '-fsanitize-coverage=<(webrtc_sanitize_coverage)' ],
+        'ldflags': [ '-fsanitize-coverage=<(webrtc_sanitize_coverage)' ],
+     }],
+     ['webrtc_sanitize_coverage!="" and OS=="mac"', {
+        'xcode_settings': {
+            'OTHER_CFLAGS': [
+               '-fsanitize-coverage=func',
+            ],
+         },
+      }],
       ['os_posix==1', {
         # For access to standard POSIXish features, use WEBRTC_POSIX instead of
         # a more specific macro.