pagetrace: fix build when experiment is on

pagetrace: fix build when experiment is on

due to a recent change, this experiment does not compile at all. This
simply fixes to pass in the new required parameter.

Change-Id: Idce0e72fa436a7acf4923717913deb3a37847fe2
Reviewed-on: https://go-review.googlesource.com/c/go/+/551415
Auto-Submit: Michael Knyszek <mknyszek@google.com>
Reviewed-by: Michael Knyszek <mknyszek@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>