scanning: Show scan failed dialog when no scanned images received

Move the logic to check the number of scanned images from the scan
preview page to when the scan completes. This fits more with the app
flow and simplifies the show boolean logic for the scan preview page.

Bug: 1059779
Change-Id: Ie742166ddd3d4ff2f79c3461428c0b88bef25544
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2587105
Commit-Queue: Gavin Williams <gavinwill@chromium.org>
Reviewed-by: Jesse Schettler <jschettler@chromium.org>
Cr-Commit-Position: refs/heads/master@{#836653}

chromeos/components/scanning/resources/scan_preview.js

@@ -102,13 +102,11 @@ Polymer({
 
   /** @private */
   onAppStateChange_() {
-    this.showScannedImages_ =
-        this.appState === AppState.DONE && this.objectUrls.length > 0;
-    this.showScanProgress_ =
-        this.appState === AppState.SCANNING || this.appState === AppState.DONE;
+    this.showScannedImages_ = this.appState === AppState.DONE;
+    this.showScanProgress_ = this.appState === AppState.SCANNING;
     this.showCancelingProgress_ = this.appState === AppState.CANCELING;
-    this.showHelperText_ =
-        !this.showScanProgress_ && !this.showCancelingProgress_;
+    this.showHelperText_ = !this.showScanProgress_ &&
+        !this.showCancelingProgress_ && !this.showScannedImages_;
   },
 
   /** @private */

chromeos/components/scanning/resources/scanning_app.js

@@ -271,7 +271,7 @@ Polymer({
    * @param {!mojoBase.mojom.FilePath} lastScannedFilePath
    */
   onScanComplete(success, lastScannedFilePath) {
-    if (!success) {
+    if (!success || this.objectUrls_.length == 0) {
       this.$.scanFailedDialog.showModal();
       return;
     }