WebApp: Move shortcuts menu downloaded icon sizes into item info struct

Previously WebApp had two separate fields:
vector<WebAppShortcutsMenuItemInfo> shortcuts_menu_item_infos_;
vector<IconSizes> downloaded_shortcuts_menu_icons_sizes_;
These were required to be the same length as they together recorded
information about the same thing (each index corresponds to an entry in
the shortcuts menu). This was both unergonomic and error-prone (see
attached bug).

This CL adds an IconSizes field to the WebAppShortcutsMenuItemInfo
struct so that there is always exactly 1 IconSizes per ItemInfo.

No functional changes.

Bug: 1427444
Change-Id: I055af4411c887124a120b912e1221d5848258975
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4619949
Reviewed-by: Dibyajyoti Pal <dibyapal@chromium.org>
Commit-Queue: Glen Robertson <glenrob@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1160992}