Update jsoncpp to remove deprecated API usage

This patch updates all usages of Json::Reader to use the new
Json::CharReader class, as Json::Reader has been deprecated.

Bug: crbug.com/983223
Change-Id: I2dde3c4fff5172a6a927def8738dd2d1b3987d74
diff --git a/src/trace_processor/export_json_unittest.cc b/src/trace_processor/export_json_unittest.cc
index ef15834..02032b7 100644
--- a/src/trace_processor/export_json_unittest.cc
+++ b/src/trace_processor/export_json_unittest.cc
@@ -86,9 +86,12 @@
   }
 
   Json::Value ToJsonValue(const std::string& json) {
-    Json::Reader reader;
+    Json::CharReaderBuilder b;
+    auto reader = std::unique_ptr<Json::CharReader>(b.newCharReader());
     Json::Value result;
-    EXPECT_TRUE(reader.parse(json, result)) << json;
+    EXPECT_TRUE(reader->parse(json.data(), json.data() + json.length(), &result,
+                              nullptr))
+        << json;
     return result;
   }