[ProfilePicker] Fix profile statistics keep-alive lifecycle on delete

When deleting a profile in the Profile Picker, `getProfileStatistics`
loads unloaded profiles asynchronously via `LoadProfileByPath()` before
initializing `profile_statistics_keep_alive_`. If the user confirms or
cancels the remove confirmation dialog before `LoadProfileByPath()`
completes (or if loading fails), `profile_statistics_keep_alive_` is
null, and an in-flight `LoadProfileByPath()` callback could subsequently
create and leak a `ScopedProfileKeepAlive`.

Cancel in-flight statistics callbacks via a dedicated `WeakPtrFactory`
in `OnJavascriptDisallowed()`, `HandleGetProfileStatistics()`,
`HandleCloseProfileStatistics()`, and `HandleRemoveProfile()`, and
tolerate a null `profile_statistics_keep_alive_` instead of
`DCHECK`/`CHECK`-failing.

Fixed: 576442273
Test: unit_tests --gtest_filter=ProfilePickerHandlerTest.*
Test: browser_tests --gtest_filter=ProfilePickerCreationFlowBrowserTest.DeleteProfile*
Change-Id: Ic9f814630fa3c51e3411aaa3184bfcccdbd083e0
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/8727574
Auto-Submit: David Roger <droger@chromium.org>
Reviewed-by: Sasha Kunitskaya <saskekun@google.com>
Reviewed-by: Monica Salama <msalama@chromium.org>
Commit-Queue: David Roger <droger@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1740656}