Files.app: Add UI to toggle and clear item selection.

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 427 matching lines @@
   </message>
   <message name="IDS_FILE_BROWSER_CREATE_FOLDER_SHORTCUT_BUTTON_LABEL" desc="Menu item label, creating a shortcut of the selected folder to the left nav.">
     Create shortcut
   </message>
   <message name="IDS_FILE_BROWSER_REMOVE_FOLDER_SHORTCUT_BUTTON_LABEL" desc="Menu item label, unpinning the selected folder from the left nav.">
     Remove shortcut
   </message>
   <message name="IDS_FILE_BROWSER_SHARE_BUTTON_LABEL" desc="Menu item label, showing dialog to share the selected file.">
     Share
   </message>
+  <message name="IDS_FILE_BROWSER_CANCEL_SELECTION_BUTTON_LABEL" desc="Label for button that unselects all selected items.">
+    Cancel selection
+  </message>
   <message name="IDS_FILE_BROWSER_CLOUD_IMPORT_BUTTON_LABEL" desc="Label for button that initiates media backup to the cloud.">
     Import <ph name="FILE_COUNT">$1<ex>5</ex></ph> files to Google Drive
   </message>
   <message name="IDS_FILE_BROWSER_CLOUD_IMPORT_ACTIVE_IMPORT_BUTTON_LABEL" desc="Message to user that there is an active import in progress.">
     Importing...
   </message>
   <message name="IDS_FILE_BROWSER_CLOUD_IMPORT_EMPTY_SCAN_BUTTON_LABEL" desc="Message to user that no new files were found, so cloud import is not possible.">
     No new media
   </message>
   <message name="IDS_FILE_BROWSER_CLOUD_IMPORT_INSUFFICIENT_SPACE_BUTTON_LABEL" desc="Message to user that there is not enough space to perform a cloud import.">
@@ skipping 5580 matching lines @@
   <message name="IDS_FLAGS_ENABLE_RESOLVE_TIMEZONE_BY_GEOLOCATION_NAME" desc="Title for the flag to enable/disable user setting to automatically update timezone by user geolocation.">
     Automatically resolve timezone by geolocation
   </message>
   <message name="IDS_FLAGS_ENABLE_RESOLVE_TIMEZONE_BY_GEOLOCATION_DESCRIPTION" desc="Description for the flag to enable/disable user setting to automatically update timezone by user geolocation.">
     If enabled, in chrome://settings a new option becomes available which allows automatically updating the timezone by the current geolocation.
   </message>
   <message name="IDS_OPTIONS_RESOLVE_TIMEZONE_BY_GEOLOCATION_DESCRIPTION" desc="Label for checkbox to allow automatic timezone update by user geolocation.">
     Automatically resolve timezone by geolocation
   </message>
 </grit-part>