IPC fuzzer: drop -fsanitize-coverage flag when dropping -fsanitize-address

Otherwise the new version of Clang warns about -fsanitize-coverage being unused.

BUG=442315, 431803

Review URL: https://codereview.chromium.org/805913002

Cr-Commit-Position: refs/heads/master@{#308376}

tools/ipc_fuzzer/mutate/mutate.gyp

@@ -24,6 +24,7 @@
             # Compiling mutate.cc with ASan takes too long, see
             # http://crbug.com/360158.
             '-fsanitize=address',
+            '-fsanitize-coverage=<(asan_coverage)',
           ],
         }],
       ],
@@ -51,6 +52,7 @@
             # Compiling generate.cc with ASan takes too long, see
             # http://crbug.com/360158.
             '-fsanitize=address',
+            '-fsanitize-coverage=<(asan_coverage)',
           ],
         }],
       ],