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Introduce DesktopPWAsWithoutExtensions command line switch.

A WebApp represents single web app.
WebApp objects are owned by WebAppRegistrar.

WebAppRegistrar is a root entity which is able to add/remove new apps.
Later WebAppRegistrar will be able:
- to iterate over all the registered apps.
- to iterate over various subsets of all registered apps (shortcut apps)
- to survive the browser (and ChromeOS) relaunch (persistence)

InstallManager is an abstract manager to plumb 3-dot menu user installation.
We will evolve its WebAppInstallManager implementation
into comprehensive install manager later.

--enable-features=DesktopPWAsWithoutExtensions will enable off-extensions
implementation for Desktop PWAs.

Bug: 891172, 871116
Change-Id: Iebcbd93cf08eec0f11c7c22dce57ded81a2a2e94
Reviewed-on: https://chromium-review.googlesource.com/c/1275468Reviewed-by: Ben Wells <benwells@chromium.org>
Reviewed-by: Trent Apted <tapted@chromium.org>
Commit-Queue: Alexey Baskakov <loyso@chromium.org>
Cr-Commit-Position: refs/heads/master@{#599887}