media: Enable encrypted media content browsertests when MojoCdm is used

media/base/key_systems.cc

Index: media/base/key_systems.cc
diff --git a/media/base/key_systems.cc b/media/base/key_systems.cc
index f71dae2f18c93cac2395f7b9590693bf63eb4ff4..66d40bf16b76c5d1066353eea79e2be91d3d0da0 100644
--- a/media/base/key_systems.cc
+++ b/media/base/key_systems.cc
@@ -19,6 +19,7 @@
 #include "media/base/key_system_names.h"
 #include "media/base/key_system_properties.h"
 #include "media/base/media.h"
+#include "media/base/media_switches.h"
 #include "ppapi/features/features.h"
 #include "media/base/media_client.h"
 #include "third_party/widevine/cdm/widevine_cdm_common.h"
@@ -404,17 +405,19 @@ void KeySystemsImpl::AddSupportedKeySystems(
     // other platforms assume the CDM can and will do anything, except for the
     // following two cases:
     // 1) AES decryptor, and
-    // 2) External Clear Key key system on Android, only enabled for testing.
+    // 2) External Clear Key key system when MojoCdm is used, only enabled for
+    //    testing.
     bool can_block = properties->UseAesDecryptor();
 #if BUILDFLAG(ENABLE_PEPPER_CDMS)
     DCHECK_EQ(properties->UseAesDecryptor(),
               properties->GetPepperType().empty());
     if (!properties->GetPepperType().empty())
       can_block = true;
-#elif defined(OS_ANDROID)
-    if (IsExternalClearKey(properties->GetKeySystemName()))
-      can_block = true;
 #endif
+    if (base::FeatureList::IsEnabled(media::kExternalClearKeyForTesting) &&

[ddorwin, 2016/10/28 19:03:47]

Does this work with ENABLE_PEPPER_CDMS? Do we expect it to be used for Pepper
ECK?

I guess use of this flag would cause us to say it is blockable. But we must
still register it as ALWAYS_ENABLED? 

[xhwang, 2016/11/03 07:14:44]

In browser_tests we do set kExternalClearKeyForTesting so this check will be
true when we run ECK tests. So for ECK can_block is always true because:
- For pepper ECK, all pepper-based key system "can block", see line 404 and 415.
- For mojo ECK, it's implemented using AesDecryptor, so it also "can block", see
line 408.

The tricky part is that pepper ECK will also execute this line, which I feel
okay. I can put this in a defined(ENABLE_MOJO_CDM) block if you like.

[ddorwin, 2016/11/10 01:24:29]

The code is inconsistent with the comment, which says this is limited to
MojoCdm. They should probably be consistent. (Refactoring this and/or the
comments, as suggested above, would also hep us avoid introducing such
inconsistencies.)

[xhwang, 2016/11/10 07:11:28]

Done.

+        IsExternalClearKey(properties->GetKeySystemName()))
+      can_block = true;
+
     if (!can_block) {
       DCHECK(properties->GetDistinctiveIdentifierSupport() ==
              EmeFeatureSupport::ALWAYS_ENABLED);