DevTools: Code Folding

This adds the ability to collapse sections of code in the Sources and
Network panels.

https://i.imgur.com/CkREJEV.png

Bug: 328431

Change-Id: I88e44d86c7fd2e6dbf076cc734aa538836516cf0
Reviewed-on: https://chromium-review.googlesource.com/c/1014386
Commit-Queue: Joel Einbinder <[email protected]>
Reviewed-by: Dmitry Gozman <[email protected]>
Reviewed-by: Erik Luo <[email protected]>
Cr-Original-Commit-Position: refs/heads/master@{#616885}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: 9e5bce11314b18020acc24e078f2ccc723be3867
diff --git a/front_end/externs.js b/front_end/externs.js
index 6f7fcbc..1353a7e 100644
--- a/front_end/externs.js
+++ b/front_end/externs.js
@@ -542,7 +542,7 @@
   undo: function() {},
   unlinkDoc: function(other) {}
 };
-/** @type {!{cursorDiv: Element, lineSpace: Element}} */
+/** @type {!{cursorDiv: Element, lineSpace: Element, gutters: Element}} */
 CodeMirror.prototype.display;
 /** @type {!{mode: string, lineWrapping: boolean}} */
 CodeMirror.prototype.options;