Add indication of camera use on SAML webcam logins

chrome/browser/resources/chromeos/login/screen_gaia_signin.html

 <link rel="import" href="chrome://oobe/custom_elements.html">
 
 <div class="step right hidden" id="gaia-signin" role="group"
     aria-live="polite" hidden>
   <div class="step-contents">
     <div id="gaia-signin-form-container">
       <webview id="signin-frame" name="signin-frame" hidden></webview>
       <offline-gaia id="offline-gaia" class="fit" hidden></offline-gaia>
       <saml-interstitial id="saml-interstitial" class="fit" hidden>
       </saml-interstitial>
     </div>
   </div>
   <div id="gaia-loading" class="step-loading" hidden>
     <throbber-notice i18n-values="text:gaiaLoading"></throbber-notice>
   </div>
   <notification-card id="gaia-whitelist-error" type="fail"
       i18n-values="button-label:tryAgainButton;
                    link-label:learnMoreButton">
   </notification-card>
   <div id="saml-notice-container" hidden>
+    <span id="saml-notice-recording-indicator" hidden>
+      <img src="chrome://theme/IDR_RESET_WARNING">

[Kevin Cernekee, 2016/05/10 21:05:42]

This is the wrong icon.

I tried IDR_TAB_RECORDING_INDICATOR but it looks like all platforms except OSX
have switched this from a raster to a vector icon, so it is unavailable on
Chrome OS.

I think I want gfx::VectorIconId::TAB_MEDIA_RECORDING but I don't know how to
get to it from the UI HTML.  Any ideas on how to proceed?

[emaxx, 2016/05/13 17:44:03]

I haven't found any way how the vector icon can be requested from an HTML page
(though probably worth asking estade@ - the /ui/gfx/vector_icons/ owner).

Otherwise the necessary IDR_TAB_RECORDING_INDICATOR raster icon could be
requested for Chrome OS too (looks like this stuff is configured in the
/chrome/app/theme/theme_resources.grd file).

[Kevin Cernekee, 2016/05/13 18:39:04]

Done.

+    </span>
     <span id="saml-notice-message"></span>
   </div>
   <navigation-bar id="gaia-navigation"></navigation-bar>
 </div>