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This CL is a simple refactor that moves WebAppInstallManager code for
enqueuing tasks that use a WebContents into a helper method. This is in
preparation for adding an update task to the queue:
https://chromium-review.googlesource.com/c/chromium/src/+/1811402

Bug: 926083
Change-Id: If66cebcbefe6ce931340c8d3c149fd42c9b36168
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1832131
Commit-Queue: Alan Cutter <alancutter@chromium.org>
Reviewed-by: Alexey Baskakov <loyso@chromium.org>
Cr-Commit-Position: refs/heads/master@{#705444}