Build with the system jsoncpp instead of the bundled one.

Adapted from Debian:
https://salsa.debian.org/chromium-team/chromium/-/blob/master/debian/patches/system/jsoncpp.patch

diff --git a/third_party/jsoncpp/BUILD.gn b/third_party/jsoncpp/BUILD.gn
--- a/third_party/jsoncpp/BUILD.gn
+++ b/third_party/jsoncpp/BUILD.gn
@@ -3,52 +3,14 @@
 # found in the LICENSE file.
 
 import("//testing/libfuzzer/fuzzer_test.gni")
+import("//build/config/linux/pkg_config.gni")
 
-config("jsoncpp_config") {
-  include_dirs = [
-    "source/include",
-    "generated",
-  ]
-
-  # TODO(crbug.com/983223): Update JsonCpp BUILD.gn to remove deprecated
-  # declaration flag.
-  # This temporary flag allowing clients to update to the new version, and then
-  # update to the new StreamWriter and CharReader classes.
-  if (!is_win || is_clang) {
-    cflags_cc = [ "-Wno-deprecated-declarations" ]
-  }
+pkg_config("jsoncpp_config") {
+  packages = [ "jsoncpp" ]
 }
 
-source_set("jsoncpp") {
-  sources = [
-    "generated/version.h",
-    "source/include/json/assertions.h",
-    "source/include/json/autolink.h",
-    "source/include/json/config.h",
-    "source/include/json/features.h",
-    "source/include/json/forwards.h",
-    "source/include/json/json.h",
-    "source/include/json/reader.h",
-    "source/include/json/value.h",
-    "source/include/json/writer.h",
-    "source/src/lib_json/json_reader.cpp",
-    "source/src/lib_json/json_tool.h",
-    "source/src/lib_json/json_value.cpp",
-    "source/src/lib_json/json_writer.cpp",
-  ]
-
+group("jsoncpp") {
   public_configs = [ ":jsoncpp_config" ]
-
-  defines = [
-    "JSON_USE_EXCEPTION=0",
-    "JSON_USE_NULLREF=0",
-  ]
-
-  include_dirs = [ "source/src/lib_json" ]
-
-  if (!is_win || is_clang) {
-    cflags_cc = [ "-Wno-implicit-fallthrough" ]
-  }
 }
 
 if (build_with_chromium) {