Add support for fetching .mjs files in DevTools

We will soon be using `acorn.mjs`, which would previously throw errors
when loading, as `type="module"` enforces strict MIMEtype checking.

Bug: 1011811
Change-Id: I1eaea02f94ebf74ee8fc32678cf29dde86c25b2c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2212124
Auto-Submit: Tim van der Lippe <tvanderlippe@chromium.org>
Commit-Queue: Andrey Kosyakov <caseq@chromium.org>
Reviewed-by: Andrey Kosyakov <caseq@chromium.org>
Cr-Commit-Position: refs/heads/master@{#771219}

chrome/browser/ui/webui/devtools_ui_data_source.cc

@@ -57,6 +57,8 @@ std::string GetMimeTypeForPath(const std::string& path) {
                             base::CompareCase::INSENSITIVE_ASCII)) {
     return "text/css";
   } else if (base::EndsWith(filename, ".js",
+                            base::CompareCase::INSENSITIVE_ASCII) ||
+             base::EndsWith(filename, ".mjs",
                             base::CompareCase::INSENSITIVE_ASCII)) {
     return "application/javascript";
   } else if (base::EndsWith(filename, ".png",