base: manually destroy raw_ptr's in GSource objects

glib manages GSource objects memory manually, and thus will not call
the object's destructor when free'ing the objects, which leads to adding
pressure to the BRP quarantine. Luckily, the dispose method, if
specified, is called just before this destruction, so that users clean
up source objects. We will use that to ensure we can track raw_ptr
refcounts.

Bug: 1445066
Change-Id: I446463e0d8370ca4da362a6d8dfccbc2ab3835a2
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4526517
Reviewed-by: danakj <danakj@chromium.org>
Commit-Queue: Paul Semel <paulsemel@chromium.org>
Reviewed-by: Bartek Nowierski <bartekn@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1147185}