Fix unconditional use of __attribute__.

(This is a merge of http://go/wvgerrit/74844)

__attribute__ is not supported on all compilers, notably it's not
supported on MSVC.  Commenting out the variable should suppress the
warning this is trying to fix.

Test: Android CDM Build
Bug: 122953649
Change-Id: I0101df1cca271415a6444e5455eb0085676bcabf

libwvdrmengine/cdm/core/include/cdm_engine.h

@@ -325,8 +325,7 @@ class CdmEngine {
   // data. Returns true if the metrics data is populated, false otherwise.
   // |engine_metrics| is owned by the caller and must not be null.
   // The CdmEngine implementation is a placeholder. Just return false.
-  virtual bool GetMetricsSnapshot(
-      __attribute__((unused)) drm_metrics::WvCdmMetrics *metrics) {
+  virtual bool GetMetricsSnapshot(drm_metrics::WvCdmMetrics* /* metrics */) {
     return false;
   }