Avoid try/catch, DOM and workers

src/ol/has.js

@@ -37,27 +37,15 @@ export const MAC = ua.indexOf('macintosh') !== -1;
  * @type {number}
  * @api
  */
-export const DEVICE_PIXEL_RATIO = (function() {
-  try {
-    return self.devicePixelRatio;
-  } catch (e) {
-    return window.devicePixelRatio || 1;
-  }
-})();
+export const DEVICE_PIXEL_RATIO = (typeof self !== 'undefined' ? self.devicePixelRatio : window.devicePixelRatio) || 1;
 
 /**
- * The execution context is a window.
+ * The execution context is a worker with OffscreenCanvas available.
  * @const
  * @type {boolean}
  */
-export const WINDOW = (function() {
-  try {
-    return 'document' in self;
-  } catch (e) {
-    // ancient browsers don't have `self`
-    return true;
-  }
-})();
+export const WORKER_OFFSCREEN_CANVAS = typeof WorkerGlobalScope !== 'undefined' && typeof OffscreenCanvas !== 'undefined' &&
+  self instanceof WorkerGlobalScope; //eslint-disable-line
 
 /**
  * Image.prototype.decode() is supported.