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A user can drag a link onto the chrome://apps page to install it as a
web app. This was previously done through the extensions system. This
updates AppLauncherHandler to use the new BMO WebApp system,
specifically the AppInstallManager.

R=calamity@chromium.org, loyso@chromium.org

Fixed: 1070136,1061286
Change-Id: I7d1ae0db4f70515efe73bb28d862dbce0ed939fd
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2148293Reviewed-by: calamity <calamity@chromium.org>
Reviewed-by: Alexey Baskakov <loyso@chromium.org>
Commit-Queue: Daniel Murphy <dmurph@chromium.org>
Cr-Commit-Position: refs/heads/master@{#761063}