Clarify that Properties::Init is called multiple times

Merge from Widevine repo of http://go/wvgerrit/53465

It may not be clear that Properties::Init is called multiple
times. This CL adds an explanation in the header file.  The function
itself is platform specific.

Test: tested as part of http://go/ag/4674759

Change-Id: I68010c594ec146e36161ae0f4f44d53caefa1896

libwvdrmengine/cdm/core/include/properties.h

@@ -31,6 +31,10 @@ typedef std::map<CdmSessionId, CdmClientPropertySet*>
 // Setter methods are provided but their only planned use is for testing.
 class Properties {
  public:
+  // Called at least once before any properties are used.  Depending on the
+  // platform, this function may be called multiple times.  It is called each
+  // time a CdmEngine is created, and when running unit tests it is called in
+  // many tests' SetUp function.
   static void Init();
 
   static inline bool oem_crypto_use_secure_buffers() {