Fix file_net_log_observer_unittest use of JSONReader

JSONReader::Read is a static method, but JSONReader::GetErrorMessage is
not. If invaild JSON is passed, GetErrorMessage() will not return
anything useful.

Bug: 1070409
Change-Id: If45ff0d665964f7b02be26a7b1af436918cbb1fb
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2227944Reviewed-by: Eric Roman <eroman@chromium.org>
Commit-Queue: Nigel Tao <nigeltao@chromium.org>
Cr-Commit-Position: refs/heads/master@{#775053}

net/log/file_net_log_observer_unittest.cc

@@ -123,12 +123,12 @@ struct ParsedNetLog {
     return ::testing::AssertionFailure() << "input is empty";
   }
 
-  base::JSONReader reader;
-  base::Optional<base::Value> container_optional = reader.Read(input);
-  if (!container_optional) {
-    return ::testing::AssertionFailure() << reader.GetErrorMessage();
+  base::JSONReader::ValueWithError parsed_json =
+      base::JSONReader::ReadAndReturnValueWithError(input);
+  if (!parsed_json.value) {
+    return ::testing::AssertionFailure() << parsed_json.error_message;
   }
-  container = std::move(*container_optional);
+  container = std::move(*parsed_json.value);
 
   if (!container.GetAsDictionary(&root)) {
     return ::testing::AssertionFailure() << "Not a dictionary";