widevine is linked against glibc and needs gcompat to work. Load the
libraries before the sandbox is started.

--- a/dom/media/gmp/GMPChild.cpp
+++ b/dom/media/gmp/GMPChild.cpp
@@ -208,6 +208,10 @@
       "libdl.so.2",
       "libpthread.so.0",
       "librt.so.1",
+      "libgcompat.so.0",
+      "ld-linux-x86-64.so.2",
+      "ld-linux-aarch64.so.1",
+      "ld-linux.so.2",
   };
 
   nsTArray<nsCString> libs;
--- a/dom/media/gmp/GMPParent.cpp
+++ b/dom/media/gmp/GMPParent.cpp
@@ -1206,6 +1206,16 @@
       // psapi.dll added for GetMappedFileNameW, which could possibly be avoided
       // in future versions, see bug 1383611 for details.
       mLibs = "dxva2.dll, ole32.dll, psapi.dll, shell32.dll, winmm.dll"_ns;
+#elif XP_LINUX
+      mLibs = "libgcompat.so.0"_ns
+#if defined(__x86_64__)
+        ", ld-linux-x86-64.so.2"_ns
+#elif defined(__aarch64__)
+        ", ld-linux-aarch64.so.1"_ns
+#elif defined(__i386__)
+        ", ld-linux.so.2"_ns
+#endif
+        ;
 #endif
       break;
 #ifdef MOZ_WMF_CDM