Gate building jsoncpp_fuzzer on |build_with_chromium|

Projects which depend on third_party/jsoncpp that are not built in
a Chromium checkout may not have the //testing/libfuzzer dependencies
needed to build jsoncpp_fuzzer.

TBR=chromium-third-party@google.com

Bug: dawn:208
Change-Id: I4664cca2a975fe52013ad7874db8e098ccbb6098
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1861014
Commit-Queue: Austin Eng <enga@chromium.org>
Reviewed-by: John Budorick <jbudorick@chromium.org>
Cr-Commit-Position: refs/heads/master@{#706932}

third_party/jsoncpp/BUILD.gn

@@ -51,16 +51,18 @@ source_set("jsoncpp") {
   }
 }
 
-fuzzer_test("jsoncpp_fuzzer") {
-  sources = [
-    "fuzzers/json_fuzzer.cc",
-  ]
+if (build_with_chromium) {
+  fuzzer_test("jsoncpp_fuzzer") {
+    sources = [
+      "fuzzers/json_fuzzer.cc",
+    ]
 
-  deps = [
-    ":jsoncpp",
-  ]
+    deps = [
+      ":jsoncpp",
+    ]
 
-  include_dirs = [ "generated" ]
+    include_dirs = [ "generated" ]
 
-  dict = "//testing/libfuzzer/fuzzers/dicts/json.dict"
+    dict = "//testing/libfuzzer/fuzzers/dicts/json.dict"
+  }
 }