[dPWA] Moving WebAppIconDownloader into the web_contents/ folder

R=dibyapal@chromium.org

Bug: 1329934,b/240995162
Change-Id: Ib05ff790821d441190429764694c4508a385daf9
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4347715


Reviewed-by: Dibyajyoti Pal <dibyapal@chromium.org>
Reviewed-by: Duncan Mercer <mercerd@google.com>
Commit-Queue: Daniel Murphy <dmurph@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1118932}