DevTools: Add tooltips describing usage numbers on Memory panel.

Change-Id: I49957e9b7c6b9bda6588823a7b864754a4de0f10
Reviewed-on: https://chromium-review.googlesource.com/1115970
Reviewed-by: Aleksey Kozyatinskiy <[email protected]>
Commit-Queue: Alexei Filippov <[email protected]>
Cr-Original-Commit-Position: refs/heads/master@{#570613}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: 73d167c577f13478d2a2b0fb039caa615bfac435
diff --git a/front_end/externs.js b/front_end/externs.js
index ae58b8b..5265820 100644
--- a/front_end/externs.js
+++ b/front_end/externs.js
@@ -674,6 +674,13 @@
 /**
  * @param {...!Node} nodes
  * @return {undefined}
+ * @see https://dom.spec.whatwg.org/#dom-parentnode-append
+ */
+Element.prototype.append = function(nodes) {};
+
+/**
+ * @param {...!Node} nodes
+ * @return {undefined}
  * @see https://dom.spec.whatwg.org/#dom-parentnode-prepend
  */
 Element.prototype.prepend = function(nodes) {};