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This is a simple refactor CL that replaces the WebappInstallSource
parameter in WebAppDataRetriever with WebAppIconDownloader::Histogram.
This removes the strong dependency on install_source in preparation for update
install tasks that don't have an install_source:
https://chromium-review.googlesource.com/c/chromium/src/+/1811402

Bug: 926083
Change-Id: I239d5094f545a5d86d2d282ed53185de935caa22
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1837573
Commit-Queue: Alan Cutter <alancutter@chromium.org>
Reviewed-by: Alexey Baskakov <loyso@chromium.org>
Cr-Commit-Position: refs/heads/master@{#705851}