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mini_installer.exe requires a temporary directory in which to write
files during processing. Historically, this has been a directory under
%TMP%. While mini_installer does its level best to delete this directory
before terminating, it cannot always do so (e.g., third-party software
may have some files open, or the machine may lose power).
mini_installer.exe scans %TMP% at process startup to delete any stale
directories from previous runs to prevent unbounded accumulation said
directories.

In this CL, mini_installer.exe now creates its work directory in the
directory containing the executable itself. In the case of installations
driven by a variant of Omaha (e.g., Google Chrome), Omaha will
eventually clean any files/directories left behind.

This CL also results in successful runs sending up either 0x10000 or
0x20000 in ExtraCode1 based on whether or not the fallback to %TMP% was
used. This will inform us as to whether or not it's safe to remove that
fallback.

A note to the authors of other Chromium forks: this change in behavior
may impact you. Audit how your variant of mini_installer.exe is run and
ensure that any files or directories left in the same directory as it
are cleaned up.

BUG=516207,1100280

Change-Id: I7a8dd7981b26dbac10553747cfe32a40fb11f37b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2351989Reviewed-by: S. Ganesh <ganesh@chromium.org>
Reviewed-by: Will Harris <wfh@chromium.org>
Commit-Queue: Greg Thompson <grt@chromium.org>
Cr-Commit-Position: refs/heads/master@{#798844}