Check JSON value instead of error_code

Checking (result.error_code != JSON_NO_ERROR) is equivalent to checking
result.value (which has a base::Optional type, convertible to bool).

The error_code enum is really a private implementation detail. Callers
don't care about it (other than it being an error or no error). Removing
references (outside of //base/json) to that enum will let us more easily
modify the //base/json implementation.

Bug: 1070409
Change-Id: I1e026b4b8e1544823b07ce2c6da9f5bd56f6c708
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2145750Reviewed-by: Ahmed Fakhry <afakhry@chromium.org>
Commit-Queue: Nigel Tao <nigeltao@chromium.org>
Cr-Commit-Position: refs/heads/master@{#759056}

ui/display/manager/json_converter_unittest.cc

@@ -49,9 +49,8 @@ TEST(JsonConverterTest, JsonFromToDisplayLayout) {
       "}";
   base::JSONReader::ValueWithError result =
       base::JSONReader::ReadAndReturnValueWithError(data, 0);
-  ASSERT_EQ(base::JSONReader::JSON_NO_ERROR, result.error_code)
-      << result.error_message << " at " << result.error_line << ":"
-      << result.error_column;
+  ASSERT_TRUE(result.value) << result.error_message << " at "
+                            << result.error_line << ":" << result.error_column;
   EXPECT_EQ(value, result.value.value());
 
   DisplayLayout read_layout;
@@ -71,9 +70,8 @@ TEST(JsonConverterTest, OldJsonToDisplayLayout) {
       "}";
   base::JSONReader::ValueWithError result =
       base::JSONReader::ReadAndReturnValueWithError(data, 0);
-  ASSERT_EQ(base::JSONReader::JSON_NO_ERROR, result.error_code)
-      << result.error_message << " at " << result.error_line << ":"
-      << result.error_column;
+  ASSERT_TRUE(result.value) << result.error_message << " at "
+                            << result.error_line << ":" << result.error_column;
 
   DisplayLayout read_layout;
   EXPECT_TRUE(JsonToDisplayLayout(result.value.value(), &read_layout));