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This CL adds a EnableAllSystemWebApps chrome feature, when enabled
SystemWebAppManager will enable and install all registered system apps,
and ignoring each system app's respective feature flag.

This is useful for testing.

This CL migrates the existing g_enable_all_system_web_apps_for_testing
to this new feature, and adds a kNumberOfRegisteredSystemApps to verify
all the apps can be installed successfully.

Bug: 1095524
Change-Id: Iba595e7d94ff20076a6bfb36e0a91cee09c897bc
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2366952Reviewed-by: Giovanni Ortuño Urquidi <ortuno@chromium.org>
Commit-Queue: Jiewei Qian  <qjw@chromium.org>
Cr-Commit-Position: refs/heads/master@{#800906}