Bug 1332530 - Don't require a SandboxStarter if MOZ_GMP_SANDBOX is not defined. r=jld

GMP gtests fail on ASAN builds now since the GMPLoader requires a sandbox
starter, and ASAN doesn't run with a GMP sandbox. So only enforce that we
need a sandbox starter if we've built with sandboxing enabled.

MozReview-Commit-ID: GptxIZ7TFIy

--HG--
extra : rebase_source : 6265f91a9c80555b63f71ac5da116450d4728df1

dom/media/gmp/GMPChild.cpp

@@ -350,11 +350,13 @@ GMPChild::AnswerStartPlugin(const nsString& aAdapter)
   InitPlatformAPI(*platformAPI, this);
 
   mGMPLoader = MakeUnique<GMPLoader>();
+#if defined(MOZ_GMP_SANDBOX)
   if (!mGMPLoader->CanSandbox()) {
     LOGD("%s Can't sandbox GMP, failing", __FUNCTION__);
     delete platformAPI;
     return IPC_FAIL_NO_REASON(this);
   }
+#endif
 
   bool isWidevine = aAdapter.EqualsLiteral("widevine");
 #if defined(MOZ_GMP_SANDBOX) && defined(XP_MACOSX)