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This change creates the DiagnosticsManager class which is responsible
for managing the lifetime of the various services used by the
Diagnostics SWA. DiagnosticsManager will be owned by the SWA's
MojoWebUIController and is brought up and torn down when the app is
opened/closed.

Bug: 1128204
Change-Id: Ie9be65e3d6ab74aa4037cc1e916cdbd3f2981da5
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2492641Reviewed-by: Bailey Berro <baileyberro@chromium.org>
Reviewed-by: Zentaro Kavanagh <zentaro@chromium.org>
Commit-Queue: Bailey Berro <baileyberro@chromium.org>
Cr-Commit-Position: refs/heads/master@{#820314}