preload-topchrome: add a tracker that tracks WebUIs per profile

This patch adds PerProfileWebUITracker that can be used to query if a
WebUI with the specified URL exists within the profile.

This class will be used by the preload manager to choose the next WebUI
to preload. For example, a strategy could be to preload only WebUIs that
currently don't exist under a profile. The integration into the preload
manager will be in following up CLs.

We probably would also want to track WebUIs per Browser. However, this
is difficult because WebUIContentsWrapper does not accept a Browser*
parameter on its construction. For now I think tracking WebUIs per
profile will be a good starting point.

Bug: 40168622
Change-Id: Ic104149a98a08b7ed98b656996641b76d2465224
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5553292
Reviewed-by: Yuheng Huang <yuhengh@chromium.org>
Code-Coverage: findit-for-me@appspot.gserviceaccount.com <findit-for-me@appspot.gserviceaccount.com>
Commit-Queue: Keren Zhu <kerenzhu@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1304544}