cros: Enable swiping to switch between preview wallpapers

1) Add event listeners to enable using swipe to easily switch between
   different wallpapers, for preview in tablet mode.

2) In |getCollectionsInfo_|, use offline image info to render the UI
   for faster loading (useful when the connection is slow).

Bug: 837355
Change-Id: I127e0fae6444939deb44e45972e91a2741c08e91
Reviewed-on: https://chromium-review.googlesource.com/1102903Reviewed-by: Alexander Alekseev <alemate@chromium.org>
Commit-Queue: Wenzhao (Colin) Zang <wzang@chromium.org>
Cr-Commit-Position: refs/heads/master@{#570246}

chrome/browser/resources/chromeos/wallpaper_manager/css/wallpaper_manager.css

@@ -383,7 +383,8 @@ body:not([surprise-me-disabled]) [visibleif~='surprise-me-disabled'] {
 #dialog-header,
 #no-images-message,
 #current-wallpaper-info-bar,
-#message-container {
+#message-container,
+#preview-canvas {
   display: none;
 }
 
@@ -728,6 +729,13 @@ html[dir='rtl'] .v2 .top-header-contents .more-options {
   display: flex;
 }
 
+.v2.preview-mode:not(.wallpaper-set-successfully) #preview-canvas {
+  -webkit-app-region: no-drag;
+  display: block;
+  height: 100%;
+  width: 100%;
+}
+
 .v2 #no-images-message {
   display: block;
   position: absolute;

chrome/browser/resources/chromeos/wallpaper_manager/main.html

@@ -188,6 +188,7 @@ found in the LICENSE file.
     </div>
   </div>
   <div id="message-container"></div>
+  <div id="preview-canvas"></div>
   <div id="spinner" hidden></div>
   <div id="daily-refresh-banner-template" hidden>
     <div class="daily-refresh-banner">