WebApp: Introduce kDesktopPWAsUnifiedInstall base feature.

--enable-features=DesktopPWAsUnifiedInstall will enable WebAppInstallManager
implementation of the abstract InstallManager interface. This implementation
doesn't use extensions at all.

Bug: 915043
Change-Id: I895739d39839aa55425c0fe190d22732e95b4858
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1584526
Reviewed-by: Dominick Ng <dominickn@chromium.org>
Commit-Queue: Alexey Baskakov <loyso@chromium.org>
Cr-Commit-Position: refs/heads/master@{#654329}
3 files changed