Do not enable sanitize-coverage on mac:clang_x64

Bug: 751212
Change-Id: I2214931d49ba27c5237582dcacdc2d85512f875e
Reviewed-on: https://chromium-review.googlesource.com/596748Reviewed-by: smut <smut@google.com>
Reviewed-by: Max Moroz <mmoroz@chromium.org>
Commit-Queue: Andrei Kapishnikov <kapishnikov@chromium.org>
Cr-Commit-Position: refs/heads/master@{#491802}

build/config/sanitizers/BUILD.gn

@@ -200,7 +200,7 @@ config("default_sanitizer_ldflags") {
       ldflags += [ "-fsanitize=vptr" ]
     }
 
-    if (use_sanitizer_coverage) {
+    if (use_sanitizer_coverage && current_toolchain != "//build/toolchain/mac:clang_x64") {
       ldflags += [ "-fsanitize-coverage=$sanitizer_coverage_flags" ]
     }
 
@@ -380,7 +380,7 @@ config("cfi_flags") {
 config("coverage_flags") {
   cflags = []
 
-  if (use_sanitizer_coverage) {
+  if (use_sanitizer_coverage  && current_toolchain != "//build/toolchain/mac:clang_x64") {
     cflags += [
       "-fsanitize-coverage=$sanitizer_coverage_flags",
       "-mllvm",