Fix instances of base::Unretained() with incomplete types on Linux.

This CL fixes all existing non-test instances on a non-official Linux
build. The followup CL enforces this for non-test code on non-official
Linux builds.

Bug: 1386249
Change-Id: I92d35b1505e388096e4ab1a64525d83aa8210a05
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4053980


Reviewed-by: danakj <danakj@chromium.org>
Owners-Override: danakj <danakj@chromium.org>
Reviewed-by: Kouhei Ueno <kouhei@chromium.org>
Commit-Queue: danakj <danakj@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1075569}