Directly write the brand file.

It's possible that there's a newer Chrome already installed on the
target Mac. In that case, the installer will skip the Chrome app
installation, and the postinstall script won't know what brand code
to write. Therefore, bake the brand code into the postinstall script.

(cherry picked from commit 5125e6132ecb47ae8bc38e6a81280d932c4da946)

Bug: 1048228
Tbr: rsesek@chromium.org
Change-Id: I57857bf5f43804fa4cfb3a552cb3a131e9ee94fc
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2036255
Reviewed-by: Robert Sesek <rsesek@chromium.org>
Commit-Queue: Avi Drissman <avi@chromium.org>
Cr-Original-Commit-Position: refs/heads/master@{#737978}
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2065851
Reviewed-by: Avi Drissman <avi@chromium.org>
Cr-Commit-Position: refs/branch-heads/3987@{#936}
Cr-Branched-From: c4e8da9871cc266be74481e212f3a5252972509d-refs/heads/master@{#722274}
3 files changed