Removing about:downloads, about:help, about:ntp, about:extensions resources.

about:downloads
  not used in Android
  downloads_ui class is excluded in chrome_browser_ui.gypi

about:help
  same as about:downloads

about:ntp
  Android uses a custom ntp and replaces ntp_resource_cache.cc with
  ntp_resource_cache_android.cc so the following are not needed:
    IDR_INCOGNITO_TAB_HTML
    IDR_GUEST_TAB_HTML
    IDR_NEW_INCOGNITO_TAB_THEME_CSS
    IDR_NEW_GUEST_TAB_THEME_CSS

about:extensions
  Android does not support extensions

BUG=326023

Review URL: https://codereview.chromium.org/143963009

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@246495 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 246495