[libfuzzer] Added dictionary for v8_json_parser_fuzzer.

Also I've manually pruned the corpus because it was too large (170k+ files).
That should reanimate the fuzzer + dictionary might be useful for JSON.

R=aizatsky@chromium.org, inferno@chromium.org, jochen@chromium.org, krasin@chromium.org
BUG=584819
TBR=aizatsky@chromium.org

Review URL: https://codereview.chromium.org/1808203002

Cr-Commit-Position: refs/heads/master@{#381707}

testing/libfuzzer/fuzzers/BUILD.gn

@@ -269,6 +269,7 @@ fuzzer_test("v8_json_parser_fuzzer") {
   deps = [
     "//v8:json_fuzzer",
   ]
+  dict = "dicts/json.dict"
 }
 
 fuzzer_test("v8_regexp_parser_fuzzer") {

testing/libfuzzer/fuzzers/dicts/json.dict

+# Copyright 2016 The Chromium Authors. All rights reserved.
+# Use of this source code is governed by a BSD-style license that can be
+# found in the LICENSE file.
+
+"{"
+"}"
+"["
+"]"
+"\""
+"'"
+"\\"
+"//"
+":"
+","
+" "
+"\\n"
+"\\r"
+"/*"
+"*/"
+"true"
+"false"
+"null"
+"\\u"
+"\\b"
+"\\f"
+"\\t"
+"."
+"e"
+"e+"
+"e-"
+"E"
+"E+"
+"E-"