Make agent shim print the app update result.

Chrome launches the agent via the registration framework with the
following arguments:
    `-productIDToUpdate com.google.Chrome -printResults YES
     -userInitiated YES`

The registration framework expects the agent print the following in STDOUT:

updateCheckSuccessful_=<YES/NO>
successfulInstallCount_=<number of updates installed>

This change make the agent print such lines when it was launched with
the above command lines arguments.

Bug: 1419881
Change-Id: I1b3226335d097696df338b6afb0abce263772009
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4327447
Reviewed-by: Joshua Pawlicki <waffles@chromium.org>
Reviewed-by: Sorin Jianu <sorin@chromium.org>
Commit-Queue: Xiaoling Bao <xiaolingbao@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1115497}