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This CL adds a new "offline_manifest" field to default web app JSON
configs.

Example:
{
  "app_url": "https://example.org/",
  "launch_container": "window",
  "user_type": ["unmanaged"],
  "offline_manifest": {
    "name": "Example",
    "start_url": "https://example.org/",
    "scope": "https://example.org/",
    "display": "standalone",
    "icon_any_pngs": ["icon.png"],
    "theme_color_argb_hex": "FFFFFFFF"
  }
}

This is to support installing default web apps on Chrome OS without
spawning a WebContents and fetching the install URL of every default app.
Additionally this is to avoid overwhelming the default web apps when
every Chrome user is migrated over to default web apps instead of
the current default Chrome apps.

Design doc:
https://docs.google.com/document/d/1pk8aLnp24AVsWwC6OaCgYZHGGqvyNxekB04rzuTGDVM/edit#heading=h.9f9lhtrsvs1k

Bug: 1119710
Change-Id: Ibe36631e4dbb540165cd67944b5ec6c8a47e8a2e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2352450
Commit-Queue: Alan Cutter <alancutter@chromium.org>
Reviewed-by: Glen Robertson <glenrob@chromium.org>
Cr-Commit-Position: refs/heads/master@{#800066}