Add mjs to Common.ResourceType._resourceTypeByExtension

Our recommendation is to use `mjs` to denote JavaScript modules as
opposed to classic scripts:
https://developers.google.com/web/fundamentals/primers/modules

Bug: chromium:857076
Change-Id: I6d3f77c62bd9a14bec484dfe897aff754301bff7
Reviewed-on: https://chromium-review.googlesource.com/1117070Reviewed-by: Paul Irish <paulirish@chromium.org>
Reviewed-by: Dmitry Gozman <dgozman@chromium.org>
Commit-Queue: Paul Irish <paulirish@chromium.org>
Cr-Commit-Position: refs/heads/master@{#571361}

third_party/blink/renderer/devtools/front_end/common/ResourceType.js

@@ -242,7 +242,7 @@ Common.ResourceType._mimeTypeByName = new Map([
 ]);
 
 Common.ResourceType._resourceTypeByExtension = new Map([
-  ['js', Common.resourceTypes.Script],
+  ['js', Common.resourceTypes.Script], ['mjs', Common.resourceTypes.Script],
 
   ['css', Common.resourceTypes.Stylesheet], ['xsl', Common.resourceTypes.Stylesheet],
 
@@ -258,8 +258,8 @@ Common.ResourceType._resourceTypeByExtension = new Map([
 
 Common.ResourceType._mimeTypeByExtension = new Map([
   // Web extensions
-  ['js', 'text/javascript'], ['css', 'text/css'], ['html', 'text/html'], ['htm', 'text/html'],
-  ['mjs', 'text/javascript'], ['xml', 'application/xml'], ['xsl', 'application/xml'],
+  ['js', 'text/javascript'], ['mjs', 'text/javascript'], ['css', 'text/css'], ['html', 'text/html'],
+  ['htm', 'text/html'], ['xml', 'application/xml'], ['xsl', 'application/xml'],
 
   // HTML Embedded Scripts, ASP], JSP
   ['asp', 'application/x-aspx'], ['aspx', 'application/x-aspx'], ['jsp', 'application/x-jsp'],