Clarify JSONValueDeserializerTest assertion

Prior to this commit, the code looked like:
    etc.Deserialize(&error_code, etc);
    ASSERT_EQ(foo, error_code);
which could be misinterpreted as testing that a side-effect of calling
Deserialize was setting error_code to foo. In fact, error_code was set
to foo before the Deserialize call, and what was actually being tested
was that Deserialize has no effect on error_code when there's no error.
That should be clearer after this commit.

Bug: 1069271
Change-Id: I20045c24c098f792a87622762d58fb085f6fbaaa
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2248395Reviewed-by: Robert Sesek <rsesek@chromium.org>
Commit-Queue: Nigel Tao <nigeltao@chromium.org>
Cr-Commit-Position: refs/heads/master@{#779796}

base/json/json_value_serializer_unittest.cc

@@ -133,12 +133,15 @@ TEST(JSONValueDeserializerTest, ReadJSONWithTrailingCommasFromString) {
   ASSERT_FALSE(value);
   ASSERT_NE(0, error_code);
   ASSERT_FALSE(error_message.empty());
-  // Repeat with commas allowed.
+  // Repeat with commas allowed. The Deserialize call shouldn't change the
+  // value of error_code. To test that, we first set it to a nonsense value
+  // (-789) and ASSERT_EQ that it remains that nonsense value.
+  error_code = -789;
   JSONStringValueDeserializer str_deserializer2(kProperJSONWithCommas,
                                                 JSON_ALLOW_TRAILING_COMMAS);
   value = str_deserializer2.Deserialize(&error_code, &error_message);
   ASSERT_TRUE(value);
-  ASSERT_EQ(JSONReader::JSON_TRAILING_COMMA, error_code);
+  ASSERT_EQ(-789, error_code);
   // Verify if the same JSON is still there.
   CheckJSONIsStillTheSame(*value);
 }
@@ -184,12 +187,15 @@ TEST(JSONValueDeserializerTest, ReadJSONWithCommasFromFile) {
   ASSERT_FALSE(value);
   ASSERT_NE(0, error_code);
   ASSERT_FALSE(error_message.empty());
-  // Repeat with commas allowed.
+  // Repeat with commas allowed. The Deserialize call shouldn't change the
+  // value of error_code. To test that, we first set it to a nonsense value
+  // (-789) and ASSERT_EQ that it remains that nonsense value.
+  error_code = -789;
   JSONFileValueDeserializer file_deserializer2(temp_file,
                                                JSON_ALLOW_TRAILING_COMMAS);
   value = file_deserializer2.Deserialize(&error_code, &error_message);
   ASSERT_TRUE(value);
-  ASSERT_EQ(JSONReader::JSON_TRAILING_COMMA, error_code);
+  ASSERT_EQ(-789, error_code);
   // Verify if the same JSON is still there.
   CheckJSONIsStillTheSame(*value);
 }