convert cc logo to png to avoid unhandled mimetype in appengine

Production AppEngine doesn't emit the proper mimetype for SVG automaticallly, requiring handling it in app code. To fix this bug, I just converted the CC logo to PNG.

BUG=330040
NOTRY=true

Review URL: https://codereview.chromium.org/119983002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@243961 0039d316-1c4b-4281-b951-d872f2087c98

chrome/common/extensions/docs/templates/private/cc.html

 <footer id="cc-info">
   <button id="send-feedback" class="google-button" data-feedback>Send Feedback</button>
   <!--p class="last-updated">Last updated August 2, 2013.</p-->
-  <p class="cc-license"><a href="http://creativecommons.org/licenses/by/3.0/"><img src="{{static}}/images/cc_by.svg" alt="Licensed Creative Commons by Attribution"></a></p>
+  <p class="cc-license"><a href="http://creativecommons.org/licenses/by/3.0/"><img src="{{static}}/images/cc_by.png" alt="Licensed Creative Commons by Attribution"></a></p>
 </footer>