[quickview] Disable image focus activation

If the user click on an image in the Quick View content area, the
the files-safe-media webview takes the focus. However, there are no
focusable elements inside the image. Blur them and put the focus back
to the quick view dialog element.

No-try: true
Test: Test coming
Bug: 1038835
Change-Id: I775122bd04397843941b078336c6fe09cf70f873
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2067839Reviewed-by: Noel Gordon <noel@chromium.org>
Commit-Queue: Noel Gordon <noel@chromium.org>
Commit-Queue: Isabella Scalzi <isabellascalzi@google.com>
Cr-Commit-Position: refs/heads/master@{#743433}

ui/file_manager/file_manager/foreground/elements/files_quick_view.js

@@ -53,6 +53,7 @@ const FilesQuickView = Polymer({
   },
 
   listeners: {
+    'files-safe-media-tap-inside': 'tapInside',
     'files-safe-media-tap-outside': 'close',
     'files-safe-media-load-error': 'loaderror',
   },
@@ -130,6 +131,13 @@ const FilesQuickView = Polymer({
     }
   },
 
+  tapInside: function(e) {
+    if (this.type === 'image') {
+      const dialog = this.shadowRoot.querySelector('#dialog');
+      dialog.focus();
+    }
+  },
+
   /**
    * @return {!FilesMetadataBox}
    */