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

media/BUILD.gn

Index: media/BUILD.gn
diff --git a/media/BUILD.gn b/media/BUILD.gn
index 3641f6a2fe4250748da71c2f5e01a49e22f67eca..d738e2c4d361375d531467c5f8c7b2bd6926d601 100644
--- a/media/BUILD.gn
+++ b/media/BUILD.gn
@@ -818,7 +818,7 @@ fuzzer_test("media_vp8_parser_fuzzer") {
     ":media",
     "//base",
   ]
-  libfuzzer_options = "filters/vp8_parser_fuzzertest.options"
+  libfuzzer_options = [ "max_len = 400000" ]
   dict = "test/vp8.dict"
 }
 
@@ -830,5 +830,5 @@ fuzzer_test("media_vp9_parser_fuzzer") {
     ":media",
     "//base",
   ]
-  libfuzzer_options = "filters/vp9_parser_fuzzertest.options"
+  libfuzzer_options = [ "max_len = 400000" ]
 }