Bug 1595093 - Make sure we always have a valid software backend, regardless of what prefs the user sets. r=lsalzman

Differential Revision: https://phabricator.services.mozilla.com/D53718

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

gfx/thebes/gfxPlatform.cpp

@@ -1941,6 +1941,9 @@ void gfxPlatform::InitBackendPrefs(BackendPrefsData&& aPrefsData) {
   swBackendBits |= BackendTypeBit(BackendType::CAIRO);
 #endif
   mSoftwareBackend = GetContentBackendPref(swBackendBits);
+  if (mSoftwareBackend == BackendType::NONE) {
+    mSoftwareBackend = BackendType::SKIA;
+  }
 
   if (XRE_IsParentProcess()) {
     gfxVars::SetContentBackend(mContentBackend);