This CL automatically elevates the metainstaller on Windows when trying
to install a system application at medium integrity. To achieve this,
the metainstaller detects when running at medium but trying to install a
system app early on, and re-launches itself at high integrity, showing
any UAC prompts necessary.
The process flow looks like this:
```
UpdaterSetup
-> Detects the need for elevation
-> Elevates UpdaterSetup and add `--expect-elevated` switch
-> UpdaterSetup installs to %ProgramFiles%
```
Bug: 1254126
Change-Id: Ic977c6a3fa6291b98ac7823c65a3592a838424e3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3546573
Reviewed-by: Joshua Pawlicki <waffles@chromium.org>
Reviewed-by: Sorin Jianu <sorin@chromium.org>
Commit-Queue: S. Ganesh <ganesh@chromium.org>
Cr-Commit-Position: refs/heads/main@{#986653}
