Add ol.BrowserFeature.DEVICE_PIXEL_RATIO constant

src/ol/browserfeature.js

@@ -11,6 +11,13 @@ ol.ASSUME_TOUCH = false;
  * @type {Object}
  */
 ol.BrowserFeature = {
+  /**
+   * The ratio between physical pixels and device-independent pixels
+   * (dips) on the device (`window.devicePixelRatio`).
+   * @type {number}
+   */
+  DEVICE_PIXEL_RATIO: goog.global.devicePixelRatio || 1,
+
   /**
    * True if browser supports touch events.
    * @type {boolean}