internal/frontend: look for strings in page content
internal/frontend: look for strings in page content

Search for desired substrings in the body of the returned page.

Fixes b/139286026.

Change-Id: Ic8a4406a399c71ad3ce4842deaa9000a13e2301a
Reviewed-on: https://team-review.git.corp.google.com/c/golang/discovery/+/526017
Reviewed-by: Julie Qiu <e6cb64861edcc38971590ff27c111e0e672de045@google.com>

golang/pkgsite,golang/pkgsite,golang/pkgsite,golang/pkgsite