Show downloads from the main profile in Incognito.

Currently, when a new incognito profile is opened, the download bubble
is empty. It should instead inherits old downloads from the main
profile. This behavior is consistent with the chrome://downloads page.

Bug: 1322557
Change-Id: Ie609528e5bbe06c53c415eaa4f4af27c2a2b3bf7
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3626297


Reviewed-by: Rohit Bhatia <bhatiarohit@google.com>
Commit-Queue: Xinghui Lu <xinghuilu@chromium.org>
Reviewed-by: Min Qin <qinmin@chromium.org>
Cr-Commit-Position: refs/heads/main@{#999901}