[libfuzzer] store custom options in .GN build target instead of a separate file.

media/test/BUILD.gn

Index: media/test/BUILD.gn
diff --git a/media/test/BUILD.gn b/media/test/BUILD.gn
index cddbc69567bf71d3132b02255f4392aff5d62fe5..68c64501b12a48e67db8504ec0057b89bdb3762e 100644
--- a/media/test/BUILD.gn
+++ b/media/test/BUILD.gn
@@ -130,5 +130,7 @@ fuzzer_test("media_pipeline_integration_fuzzer") {
     "//testing/gmock",
     "//ui/gfx:test_support",
   ]
-  libfuzzer_options = "media_pipeline_integration_fuzzer.options"
+
+  # This is done to avoid DEATH of ASan with "Thread limit exceeded" error.
+  libfuzzer_options = [ "runs=500000" ]
 }