Add Fabric.js as a front-end module

This CL introduces Fabric.js to the DevTools front-end. As a part of
the WebAudio Devtools project, the graph visualizer needs Fabric.js
for the fast 2D rendering.

The complete working prototype:
https://chromium-review.googlesource.com/c/devtools/devtools-frontend/+/1893911

Bug: 1022873
Change-Id: I57d6fc2b8d06c3adfac52e100f9307829d6a1c65
Reviewed-on: https://chromium-review.googlesource.com/c/devtools/devtools-frontend/+/1906741
Commit-Queue: Hongchan Choi <[email protected]>
Reviewed-by: Tim van der Lippe <[email protected]>
Reviewed-by: Chris Palmer <[email protected]>
diff --git a/front_end/externs.js b/front_end/externs.js
index 91d4cc8..e6ec1ca 100644
--- a/front_end/externs.js
+++ b/front_end/externs.js
@@ -1542,3 +1542,5 @@
   close() {
   }
 }
+
+const fabric = {};