By pure coincidence I just happened to be annoyed by that. :)

I believe the following does the trick:

https://chromium-review.googlesource.com/651612

https://chromium-review.googlesource.com/c/chromiumos/chromite/+/651612

The following revision refers to this bug:
  https://chromium.googlesource.com/chromiumos/chromite/+/f8fd58541d8d4252b65d24ac0d09001aa5568ea0

commit f8fd58541d8d4252b65d24ac0d09001aa5568ea0
Author: Alexandre Courbot <acourbot@chromium.org>
Date: Wed Sep 06 10:15:23 2017

CHROME_LKGM: allow trailing newline in file

Many editors automatically add a trailing newline at the end of files.
This makes manually editing CHROME_LKGM hazardous, since cros chrome-sdk
would then fail with the following error:

   ToGNString
       raise GNException("Trying to print a string with a newline in it.")

Strip trailing newline/whitespaces characters in CHROME_LKGM to cover
that case.

BUG= chromium:762146 
TEST=tried cros chrome-sdk with a CHROME_LKGM ending by a newline,
confirmed it worked.

Change-Id: I6f5daeedf1f7f852b38ea963e5d649a8967fab82
Reviewed-on: https://chromium-review.googlesource.com/651612
Commit-Ready: Alexandre Courbot <acourbot@chromium.org>
Tested-by: Alexandre Courbot <acourbot@chromium.org>
Reviewed-by: Achuith Bhandarkar <achuith@chromium.org>
Reviewed-by: Steven Bennetts <stevenjb@chromium.org>

[modify] https://crrev.com/f8fd58541d8d4252b65d24ac0d09001aa5568ea0/cli/cros/cros_chrome_sdk.py

Fix is merged, closing.