[fidl] Initialize the ReportingMode in the ErrorReporter

When enabling the new PM, I noticed that the `using_tests` in
`zircon/system/utest/fidl-compiler/using_tests.cc` was failing because
no errors were reported. This was because the reporting mode was not
initialized, allowing for the `library.Compile()` to still return false,
while not reporting any errors.

Change-Id: If7d926f7f41c508312719124b504f7ca0c8b68ea
diff --git a/zircon/tools/fidl/include/fidl/error_reporter.h b/zircon/tools/fidl/include/fidl/error_reporter.h
index 94f2db7..442028f 100644
--- a/zircon/tools/fidl/include/fidl/error_reporter.h
+++ b/zircon/tools/fidl/include/fidl/error_reporter.h
@@ -82,7 +82,7 @@
   void AddError(std::string formatted_message);
   void AddWarning(std::string formatted_message);
 
-  ReportingMode mode_;
+  ReportingMode mode_ = ReportingMode::kReport;
   bool warnings_as_errors_;
   std::vector<std::string> errors_;
   std::vector<std::string> warnings_;