AdmobTest/Assets/GoogleMobileAds/link.xml

<linker>
  <assembly fullname="GoogleMobileAds.iOS" ignoreIfMissing="1">
    <namespace fullname="GoogleMobileAds" preserve="all" />
    <namespace fullname="GoogleMobileAds.iOS" preserve="all" />
  </assembly>
<assembly fullname="GoogleMobileAds.Android" ignoreIfMissing="1">
    <namespace fullname="GoogleMobileAds" preserve="all" />
    <namespace fullname="GoogleMobileAds.Android" preserve="all" />
  </assembly>
</linker>