Use GetLastUsedProfileIfLoaded() in mirroring_activity

GetLastUsedProfile() is currently used in mirroring_activity for metrics
reporting. This function may cause a profile to get loaded synchronously
if none are loaded, and this seems to have started causing crashes
recently in some cases. So, this change updates the call to be
GetLastUsedProfileIfLoaded(), which does not block but may return
nullptr. This is OK in this case since we are just using it for metrics
reporting, and it is alright if we miss some cases where the pause_count
is 0.

Bug: b/368150032
Change-Id: I0179f980b893f57040625ddf31623f91b6736b2e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5879119
Reviewed-by: Takumi Fujimoto <takumif@chromium.org>
Commit-Queue: Benjamin Zielinski <bzielinski@google.com>
Cr-Commit-Position: refs/heads/main@{#1360241}