Remove resource map from PageLoadMetricsTestWaiter

We currently maintain this resource map so that tests can define
their own Testwaiter and wait on different conditions. This code is
confusing and the same functionality can be achieved without
holding all resources loaded by the page in a map.

Bug: 943598
Change-Id: If11f599079d85875b5b368f7a2021daf45765a6f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1529183
Commit-Queue: John Delaney <johnidel@chromium.org>
Reviewed-by: Charlie Harrison <csharrison@chromium.org>
Cr-Commit-Position: refs/heads/master@{#642064}