DevTools: highlight *.mjs as javascript files

This patch starts treating *.mjs files as javascript files.

BUG=781088
R=dgozman

Change-Id: I58222d6b7ad3d1c3c3442048da9cf46692b23172
Reviewed-on: https://chromium-review.googlesource.com/794349Reviewed-by: Dmitry Gozman <dgozman@chromium.org>
Commit-Queue: Andrey Lushnikov <lushnikov@chromium.org>
Cr-Commit-Position: refs/heads/master@{#519870}

third_party/WebKit/Source/devtools/front_end/common/ResourceType.js

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

third_party/WebKit/Source/devtools/front_end/persistence/FileSystemWorkspaceBinding.js

@@ -195,7 +195,7 @@ Persistence.FileSystemWorkspaceBinding._styleSheetExtensions = new Set(['css', '
 Persistence.FileSystemWorkspaceBinding._documentExtensions = new Set(['htm', 'html', 'asp', 'aspx', 'phtml', 'jsp']);
 Persistence.FileSystemWorkspaceBinding._scriptExtensions = new Set([
   'asp', 'aspx', 'c', 'cc', 'cljs', 'coffee', 'cpp', 'cs', 'dart', 'java', 'js',
-  'jsp', 'jsx',  'h', 'm',  'mm',   'py',     'sh',  'ts', 'tsx',  'ls'
+  'jsp', 'jsx',  'h', 'm',  'mjs',  'mm',     'py',  'sh', 'ts',   'tsx',  'ls'
 ]);
 
 Persistence.FileSystemWorkspaceBinding._imageExtensions = Persistence.IsolatedFileSystem.ImageExtensions;