Use actual weak ptr in TestMetricsServiceClient::uploader_

Avoids a dangling raw_ptr<> warning, and makes implementation
safely match the comment.

Bug: 1401495
Change-Id: I700361ab3d05beff0a77e5437049f4abbe8a66ac
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4234350


Commit-Queue: Tom Sepez <tsepez@chromium.org>
Reviewed-by: Luc Nguyen <lucnguyen@google.com>
Cr-Commit-Position: refs/heads/main@{#1103460}