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This changes ShortcutHelper to download the splash image for a webapp
while the webapp is being installed, rather than after. If the splash
image is downloaded before the webapp installation process finishes,
the splash image will be stored in a local map until it can be
transferred to the WebappDataStorage object.

This simplifies the code and fixes a potential UAF.

Bug: 1147913
Change-Id: I9405ee9cc304eaf8b9ff2bbaca17c1c63cfd6e65
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2533031
Commit-Queue: Evan Stade <estade@chromium.org>
Reviewed-by: Dominick Ng <dominickn@chromium.org>
Cr-Commit-Position: refs/heads/master@{#826837}