Bug 831313 - Bail out from rendering rather than aborting if we fail to allocate shared memory for a layers surface. r=roc

gfx/layers/basic/BasicThebesLayer.cpp

@@ -461,7 +461,8 @@ BasicShadowableThebesLayer::CreateBuffer(Buffer::ContentType aType,
     PR_snprintf(buf, buflen,
                 "creating ThebesLayer 'back buffer' failed! width=%d, height=%d, type=%x",
                 aSize.width, aSize.height, int(aType));
-    NS_RUNTIMEABORT(buf);
+    NS_ERROR(buf);
+    return nullptr;
   }
 
   NS_ABORT_IF_FALSE(!mIsNewBuffer,