[deps] Copy source map for 'marked' to gen/ build output directory

This makes the source map available when using DevTools-on-DevTools
with --custom-devtools-frontend. We don't actually ship the source map,
the devtools_pre_built GN rule filters out source map files for GRD
bundling.

[email protected]

Bug: None
Change-Id: Id65e96195d5a5cba48ab8b6417d62a6f88e25774
Reviewed-on: https://chromium-review.googlesource.com/c/devtools/devtools-frontend/+/5952527
Auto-Submit: Simon Zünd <[email protected]>
Commit-Queue: Simon Zünd <[email protected]>
Commit-Queue: Alex Rudenko <[email protected]>
Reviewed-by: Alex Rudenko <[email protected]>

Author: szuend

front_end/third_party/marked/BUILD.gn

@@ -14,6 +14,7 @@ devtools_pre_built("marked") {
     "marked-tsconfig.json",
     "package/lib/marked.esm.d.ts",
     "package/lib/marked.esm.js",
+    "package/lib/marked.esm.js.map",
   ]
 }