Fixed test and log formatting for CdmResponseType.

[ Merge of http://go/wvgerrit/168397 ]

When CdmResponseType (enum) was transformed to CdmResponseType
(struct), the test printers where not updated to print the result
of failed comparisons.  In addition, several logs statements were
updated haphazardly, leaving inconsistencies and potential
compiler-specific behavior.

This CL replaces CdmResponseType std::string operator with a ToString()
method.  This is to make it consistent with Google's C++ style guide
on conversion operators vs methods.  The string conversion function is
now defined in wv_cdm_types.cpp instead of inline in the header file.

The PrintTo function has been implemented along with the other CDM
test printers in test_printers.cpp.

Bug: 273989359
Test: run_x86_64_tests
Test: MediaDrmParameterizedTests on redfin
Test: Forrest drm_compliance
Change-Id: Ibfaa17029046b75b1c8c278f7bd7e04a24379848

libwvdrmengine/cdm/metrics/src/attribute_handler.cpp

@@ -20,7 +20,7 @@ template <>
 void SetAttributeField<drm_metrics::Attributes::kErrorCodeFieldNumber,
                        CdmResponseType>(const CdmResponseType& cdm_error,
                                         drm_metrics::Attributes* attributes) {
-  attributes->set_error_code(cdm_error.Enum());
+  attributes->set_error_code(cdm_error.code());
 }
 
 template <>