Use Shift-Meta-drag for rectangular selection on Chrome OS

Issue codemirror#3956

Author: marijnh

lib/codemirror.js

@@ -41,6 +41,7 @@
   // This is woefully incomplete. Suggestions for alternative methods welcome.
   var mobile = ios || /Android|webOS|BlackBerry|Opera Mini|Opera Mobi|IEMobile/i.test(userAgent);
   var mac = ios || /Mac/.test(platform);
+  var chromeOS = /\bCrOS\b/.test(userAgent);
   var windows = /win/i.test(platform);
 
   var presto_version = presto && userAgent.match(/Version\/(\d*\.\d*)/);
@@ -3680,7 +3681,7 @@
       ourIndex = doc.sel.primIndex;
     }
 
-    if (e.altKey) {
+    if (chromeOS ? e.shiftKey && e.metaKey : e.altKey) {
       type = "rect";
       if (!addNew) ourRange = new Range(start, start);
       start = posFromMouse(cm, e, true, true);