InstallableManager now supports webp icons for desktop PWA installs,
in addition to png and svg icons.

TBR=mmenke@chromium.org

Bug: 466958
Change-Id: I47fe3ae5c4d96fabba7dbc2d839d8e299df47355
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1942904
Auto-Submit: Eric Willigers <ericwilligers@chromium.org>
Commit-Queue: Dominick Ng <dominickn@chromium.org>
Reviewed-by: Dominick Ng <dominickn@chromium.org>
Reviewed-by: Alan Cutter <alancutter@chromium.org>
Cr-Commit-Position: refs/heads/master@{#721884}