Add a cancel button to the details panel...visible when import is active.

chrome/app/chromeos_strings.grdp

 <?xml version="1.0" encoding="utf-8"?>
 <!-- ChromeOS-specific strings (included from generated_resources.grd).
      Everything in this file is wrapped in <if expr="chromeos">. -->
 <grit-part>
   <message name="IDS_EMPTY_STRING" desc="Empty string, exist only to make code generic. No translation required.">
   </message>
   <message name="IDS_CHROMEBOOK" desc="The localized name for Chromebook.">
     Chromebook
   </message>
   <message name="IDS_CHROMEBOX" desc="The localized name for Chromebox">
@@ skipping 463 matching lines @@
   </message>
   <message name="IDS_FILE_BROWSER_CLOUD_IMPORT_START" desc="Tooltip of the toolbar button that starts backup.">
     Backup your photos and videos
   </message>
   <message name="IDS_FILE_BROWSER_CLOUD_IMPORT_SHOW_DETAILS" desc="Tooltip for button that shows details panel.">
     Show Cloud backup details
   </message>
   <message name="IDS_FILE_BROWSER_CLOUD_IMPORT_COMMAND" desc="Label of button on details panel that invokes cloud import feature.">
     Back up
   </message>
+  <message name="IDS_FILE_BROWSER_CLOUD_IMPORT_CANCEL_COMMAND" desc="Label of button on details panel that cancels an active import.">
+    Cancel
+  </message>
   <message name="IDS_FILE_BROWSER_CLOUD_IMPORT_STATUS_SCANNING" desc="Cloud import scanning status message.">
     Scanning your media device...
     <ph name="LINE_BREAK1">&lt;br&gt;</ph>
     <ph name="FILE_COUNT">$1<ex>5</ex></ph> new photos found
   </message>
   <message name="IDS_FILE_BROWSER_CLOUD_IMPORT_TOOLTIP_SCANNING" desc="Cloud import scanning tooltip.">
     Scanning your media device...
   </message>
   <message name="IDS_FILE_BROWSER_CLOUD_IMPORT_STATUS_NO_MEDIA" desc="Cloud import no-new-media-found status message. Shown after backup is completed and when a search for new files to backup produces no result.">
     All backed up to <ph name="BEGIN_LINK">&lt;span class='destination-link'&gt;</ph>Google Drive!<ph name="END_LINK">&lt;/span&gt;</ph>
@@ skipping 5614 matching lines @@
   <message name="IDS_OPTIONS_RESOLVE_TIMEZONE_BY_GEOLOCATION_DESCRIPTION" desc="Label for checkbox to allow automatic timezone update by user geolocation.">
     Set time zone automatically using your location
   </message>
   <message name="IDS_FLAGS_ENABLE_FIREWALL_HOLE_PUNCHING_NAME" desc="Name of the option to enable punching holes in the Chrome OS firewall.">
     Enable firewall hole punching.
   </message>
   <message name="IDS_FLAGS_ENABLE_FIREWALL_HOLE_PUNCHING_DESCRIPTION" desc="Description of the option to enable punching holes in the Chrome OS firewall.">
     Enable punching holes in the Chrome OS firewall for Chrome Apps that open a TCP/IP server socket or UDP socket.
   </message>
 </grit-part>