Gallery.app: Add shortcut keys for zooming in the slide mode.

ui/file_manager/gallery/js/image_editor/viewport.js

Index: ui/file_manager/gallery/js/image_editor/viewport.js
diff --git a/ui/file_manager/gallery/js/image_editor/viewport.js b/ui/file_manager/gallery/js/image_editor/viewport.js
index df53d7476c4b98db1aa87ea62e9dbaea90dfd9cf..d4122162f877bb08cc88e8b90001e77aa9c4bfae 100644
--- a/ui/file_manager/gallery/js/image_editor/viewport.js
+++ b/ui/file_manager/gallery/js/image_editor/viewport.js
@@ -30,6 +30,21 @@ function Viewport() {
    * @private
    */
   this.scale_ = 1;
+
+  /**
+   * Index of zoom ratio. 0 is "zoom ratio = 1".
+   * @type {number}
+   * @private
+   */
+  this.zoomIndex_ = 0;
+
+  /**
+   * Zoom ratio specified by user operations.
+   * @type {number}
+   * @private
+   */
+  this.zoom_ = 1;
+
   this.offsetX_ = 0;
   this.offsetY_ = 0;
 
@@ -39,6 +54,22 @@ function Viewport() {
 }
 
 /**
+ * Zoom ratios.
+ *
+ * @type {Object.<string, number>}
+ * @const
+ */
+Viewport.ZOOM_RATIOS = Object.freeze({
+  '3': 3,
+  '2': 2,
+  '1': 1.5,
+  '0': 1,
+  '-1': 0.75,
+  '-2': 0.5,
+  '-3': 0.25
+});
+
+/**
  * @param {number} width Image width.
  * @param {number} height Image height.
  */
@@ -57,6 +88,28 @@ Viewport.prototype.setScreenSize = function(width, height) {
 };
 
 /**
+ * Sets the new zoom ratio.
+ * @param {number} zoomIndex New zoom index.
+ */
+Viewport.prototype.setZoomIndex = function(zoomIndex) {
+  // Ignore the invalid zoomIndex.
+  if (!Viewport.ZOOM_RATIOS[zoomIndex.toString()])
+    return;
+
+  this.zoomIndex_ = zoomIndex;
+  this.zoom_ = Viewport.ZOOM_RATIOS[zoomIndex];
+  this.invalidateCaches();
+};
+
+/**
+ * Returns the current zoom index.
+ * @return {number} Zoon index.
+ */
+Viewport.prototype.getZoomIndex = function() {
+  return this.zoomIndex_;
+};
+
+/**
  * Set the size by an HTML element.
  *
  * @param {HTMLElement} frame The element acting as the "screen".
@@ -388,7 +441,7 @@ Viewport.prototype.update = function() {
  * @return {string} Transformation description.
  */
 Viewport.prototype.getTransformation = function() {
-  return 'scale(' + (1 / window.devicePixelRatio) + ')';
+  return 'scale(' + (1 / window.devicePixelRatio * this.zoom_) + ')';
 };
 
 /**