Add an assertion against building fuzz targets with code coverage on Mac.

Bug: 790747, 759794
Change-Id: Ic95c5217c6781ed5a8628200397a128f418d1a0d
Reviewed-on: https://chromium-review.googlesource.com/1024444Reviewed-by: Abhishek Arya <inferno@chromium.org>
Reviewed-by: Yuke Liao <liaoyuke@chromium.org>
Commit-Queue: Max Moroz <mmoroz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#553148}

testing/libfuzzer/fuzzer_test.gni

@@ -4,6 +4,7 @@
 
 # Defines fuzzer_test.
 #
+import("//build/config/coverage/coverage.gni")
 import("//build/config/features.gni")
 import("//build/config/sanitizers/sanitizers.gni")
 import("//testing/test.gni")
@@ -27,6 +28,10 @@ import("//testing/test.gni")
 # config (.options file) file would be generated or modified in root output
 # dir (next to test).
 template("fuzzer_test") {
+  assert(!(is_mac && use_clang_coverage && use_fuzzing_engine),
+         "Code Coverage for fuzz targets does not work on Mac until " +
+             "crbug.com/790747 is resolved.")
+
   if (!disable_libfuzzer && (use_fuzzing_engine || use_drfuzz || is_linux)) {
     assert(defined(invoker.sources), "Need sources in $target_name.")