Bug 1574327. Add a pref that we can use to disable d3d11 blacklist. r=aosmond

We'll use this to enable WARP on CI.

Differential Revision: https://phabricator.services.mozilla.com/D42221

--HG--
extra : moz-landing-system : lando

gfx/thebes/gfxWindowsPlatform.cpp

@@ -1368,7 +1368,8 @@ void gfxWindowsPlatform::InitializeD3D11Config() {
 
   nsCString message;
   nsCString failureId;
-  if (!gfxPlatform::IsGfxInfoStatusOkay(nsIGfxInfo::FEATURE_DIRECT3D_11_LAYERS,
+  if (StaticPrefs::layers_d3d11_enable_blacklist_AtStartup() &&
+      !gfxPlatform::IsGfxInfoStatusOkay(nsIGfxInfo::FEATURE_DIRECT3D_11_LAYERS,
                                         &message, failureId)) {
     d3d11.Disable(FeatureStatus::Blacklisted, message.get(), failureId);
   }

modules/libpref/init/StaticPrefList.yaml

@@ -3721,6 +3721,11 @@
   value: false
   mirror: once
 
+- name: layers.d3d11.enable-blacklist
+  type: bool
+  value: true
+  mirror: once
+
 - name: layers.deaa.enabled
   type: RelaxedAtomicBool
   value: false