As a quick workaround (if you don't mind dropping other --vmodule patterns), I think you can just add "!--vmodule" at the beginning of chrome_dev.conf to clear the patterns that are hardcoded in session_manager.

Sent https://chromium-review.googlesource.com/c/458797/ for review.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromiumos/platform2/+/ff65302b1886220da7782b6c06b7fc11d0a69e39

commit ff65302b1886220da7782b6c06b7fc11d0a69e39
Author: Daniel Erat <derat@chromium.org>
Date: Sat Mar 25 02:38:22 2017

libchromeos-ui: Prepend patterns to --vmodule.

Make ChromiumCommandBuilder::AddVmodulePattern() and the
"vmodule=" directive in /etc/chrome_dev.conf prepend
patterns to the --vmodule flag instead of appending them.

Chrome uses the first matching pattern that it finds, so
without prepending there's no easy way to use
chrome_dev.conf to override a hardcoded pattern that was
previously added via AddVmodulePattern().

BUG= chromium:704696 
TEST=updated tests; also manually checked that i can now use
     /etc/chrome_dev.conf to increase the verbosity of a
     module that's already matched by a pattern added by
     session_manager's chrome_setup.cc file

Change-Id: If6023450c0a6160406de08d17eb596828591a4c9
Reviewed-on: https://chromium-review.googlesource.com/458797
Commit-Ready: Dan Erat <derat@chromium.org>
Tested-by: Dan Erat <derat@chromium.org>
Reviewed-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Sergey Poromov <poromov@chromium.org>

[modify] https://crrev.com/ff65302b1886220da7782b6c06b7fc11d0a69e39/libchromeos-ui/chromeos/ui/chromium_command_builder.cc
[modify] https://crrev.com/ff65302b1886220da7782b6c06b7fc11d0a69e39/libchromeos-ui/chromeos/ui/chromium_command_builder.h
[modify] https://crrev.com/ff65302b1886220da7782b6c06b7fc11d0a69e39/libchromeos-ui/chromeos/ui/chromium_command_builder_unittest.cc