Add comments to unit tests

Merge from Widevine repo of http://go/wvgerrit/73383

Partners have been requesting more explanation for what each test is
doing.  This is an attempt to clarify some of the tests.

Bug: 119640588
Test: unit tests
Change-Id: I270203b4e8fa7e65abb55297788e4d55856e7dcd

libwvdrmengine/oemcrypto/test/oec_device_features.h

@@ -8,8 +8,13 @@
 
 namespace wvoec {
 
+// Keeps track of which features are supported by the version of OEMCrypto being
+// tested.  See the integration guide for a list of optional features.
 class DeviceFeatures {
  public:
+  // There are several possible methods used to derive a set of known session
+  // keys.  For example, the test can install a known test keybox, or it can
+  // parse the OEM certificate.
   enum DeriveMethod {      // Method to use derive session keys.
     NO_METHOD,             // Cannot derive known session keys.
     LOAD_TEST_KEYBOX,      // Call LoadTestKeybox before deriving keys.
@@ -32,15 +37,24 @@ class DeviceFeatures {
   uint32_t api_version;
   OEMCrypto_ProvisioningMethod provisioning_method;
 
+  // This should be called from the test program's main procedure.
   void Initialize(bool is_cast_receiver, bool force_load_test_keybox);
+  // Generate a GTest filter of tests that should not be run. This should be
+  // called after Initialize.  Tests are filtered out based on which features
+  // are not supported.  For example, a device that uses Provisioning 3.0 will
+  // have all keybox tests filtered out.
   std::string RestrictFilter(const std::string& initial_filter);
 
  private:
+  // Decide which method should be used to derive session keys, based on
+  // supported featuers.
   void PickDerivedKey();
-  bool IsTestKeyboxInstalled();
+  // Add a GTest filter restriction to the current filter.
   void FilterOut(std::string* current_filter, const std::string& new_filter);
 };
 
+// There is one global set of features for the version of OEMCrypto being
+// tested.  This should be initialized in the test program's main procedure.
 extern DeviceFeatures global_features;
 const char* ProvisioningMethodName(OEMCrypto_ProvisioningMethod method);