Back button for the WebView-based sign-in flow.

chrome/app/theme/theme_resources.grd

 <?xml version="1.0" encoding="utf-8"?>
 <!-- This grd file contains images that are pre-scaled for device scale
      factors. The image returned by |base::ResourceBundle::GetImageNamed|
      will contain multiple |gfx:ImageSkiaReps| for each scale factors. The
      |fallback_to_low_resolution| attribute controls behavior when
      an image file is missing for target scale factor. If true, chrome
      will automatically scale the 1x image to the target scale factor.
      If false, build will fail due to missing resources. This is
      currently set to true for all resources, but the default will
      become false in near future.
@@ skipping 425 matching lines @@
       <structure type="chrome_scaled_image" name="IDR_KEYWORD_SEARCH_MAGNIFIER" file="common/keyword_search_magnifier.png" />
       <structure type="chrome_scaled_image" name="IDR_KILLED_TAB" file="common/killtab.png" />
       <structure type="chrome_scaled_image" name="IDR_LAPTOP_FAVICON" file="common/favicon_laptop.png" />
       <if expr="enable_service_discovery">
         <structure type="chrome_scaled_image" name="IDR_LOCAL_DISCOVERY_CLOUDPRINT_ICON" file="common/cloudprint.png" />
       </if>
       <structure type="chrome_scaled_image" name="IDR_LOCATION_BAR_HTTP" file="common/location_bar_http.png" />
       <if expr="chromeos">
         <!-- TODO(nkostylev): This resource needs to be removed when cros login code is moved to ash. -->
         <structure type="chrome_scaled_image" name="IDR_LAUNCHER_BACKGROUND" file="cros/launcher_background.png" />
+        <structure type="chrome_scaled_image" name="IDR_LOGIN_BACK_BUTTTON" file="cros/login_back_button.png" />
+        <structure type="chrome_scaled_image" name="IDR_LOGIN_BACK_BUTTTON_MASKED" file="cros/login_back_button_masked.png" />
         <structure type="chrome_scaled_image" name="IDR_LOGIN_CLOSE_BUTTTON" file="cros/login_close_button.png" />
         <structure type="chrome_scaled_image" name="IDR_LOGIN_CLOSE_BUTTTON_MASKED" file="cros/login_close_button_masked.png" />
         <structure type="chrome_scaled_image" name="IDR_LOGIN_CLOSE_BUTTTON_ON_WHITE" file="cros/login_close_button_on_white.png" />
         <structure type="chrome_scaled_image" name="IDR_LOGIN_PASSWORD_CAPS_LOCK" file="cros/login_password_capslock.png" />
       </if>
       <if expr="chromeos">
         <structure type="chrome_scaled_image" name="IDR_KIOSK_APP_USER_POD_ICON" file="cros/kiosk_app_user_pod_icon.png" />
       </if>
       <structure type="chrome_scaled_image" name="IDR_LEGACY_SUPERVISED_USER_ICON" file="common/limited_user.png" />
       <structure type="chrome_scaled_image" name="IDR_SUPERVISED_USER_LABEL_BOTTOM" file="common/supervised_user_theme/label_bg_bottom.png" />
@@ skipping 601 matching lines @@
       <!-- People search images -->
       <structure type="chrome_scaled_image" name="IDR_PEOPLE_SEARCH_ACTION_EMAIL" file="common/mail.png" />
       <structure type="chrome_scaled_image" name="IDR_PEOPLE_SEARCH_ACTION_EMAIL_HOVER" file="common/mail_hover.png" />
       <structure type="chrome_scaled_image" name="IDR_PEOPLE_SEARCH_ACTION_EMAIL_PRESSED" file="common/mail_pressed.png" />
       <structure type="chrome_scaled_image" name="IDR_PEOPLE_SEARCH_ACTION_CHAT" file="common/chat.png" />
       <structure type="chrome_scaled_image" name="IDR_PEOPLE_SEARCH_ACTION_CHAT_HOVER" file="common/chat_hover.png" />
       <structure type="chrome_scaled_image" name="IDR_PEOPLE_SEARCH_ACTION_CHAT_PRESSED" file="common/chat_pressed.png" />
     </structures>
   </release>
 </grit>