Note that if the argument "disable_arc_opt_in" is true in ArcTest.initialize() then it will sign in as arcplusplustest@gmail.com by default.

This shows that the argument "disable_arc_opt_in" will be forwarded to chrome.Chrome():
http://cs/chromeos_internal/src/third_party/autotest/files/client/common_lib/cros/arc.py?l=486&rcl=b4328a6cbaea2cd9d4d700df3547ebf7dbeda8d8

This shows that if "disable_arc_opt_in" is false and ARC is available then arc_util.set_browser_options_for_opt_in() will be called:
http://cs/chromeos_public/src/third_party/autotest/files/client/common_lib/cros/chrome.py?l=137&rcl=b4328a6cbaea2cd9d4d700df3547ebf7dbeda8d8

This shows that the account arcplusplustest@gmail.com will be used for sign in:
http://cs/chromeos_public/src/third_party/autotest/files/client/common_lib/cros/arc_util.py?l=138&rcl=b4328a6cbaea2cd9d4d700df3547ebf7dbeda8d8

The fields "email_id" and "password" under ArcTest are not even used anywhere:
https://cs.corp.google.com/chromeos_internal/src/third_party/autotest/files/client/common_lib/cros/arc.py?q=email_id

These fields were added in https://chrome-internal-review.googlesource.com/c/chromeos/autotest-cheets/+/250036

Note the description for email_id says that it is "only used when account_util is set to true".

However, it was not cleaned up when account_util was removed in https://chrome-internal-review.googlesource.com/c/chromeos/autotest-cheets/+/270804

The following revision refers to this bug:
  https://chrome-internal.googlesource.com/chromeos/autotest-cheets/+/9096e22b06319f229e2c73e61d05423de51b82f5

commit 9096e22b06319f229e2c73e61d05423de51b82f5
Author: Ted Lai <tedlai@google.com>
Date: Wed Aug 30 20:04:31 2017

The following revision refers to this bug:
  https://chromium.googlesource.com/chromiumos/third_party/autotest/+/05fe230e4d13d027842ee0f71fb4143dc282f420

commit 05fe230e4d13d027842ee0f71fb4143dc282f420
Author: Ted Lai <tedlai@google.com>
Date: Mon Sep 04 03:57:38 2017

Remove the defunct arguments email_id and password in arc_setup().

Also, expose the arguments username and password in initialize() for
clarity.

Verified that the defunct arguments are not used anywhere by this
command:
$ grep -rnw
'/usr/local/google/home/tedlai/chromiumos/src/third_party/' -e
'email_id='

BUG= chromium:740423 
TEST=test_that <DUT_IP> cheets_AuthPerf, cheets_MailBench,
cheets_PlayStoreOptIn, cheets_PlayStoreTest.

Change-Id: I785ba21020b86acd94e2f4c9194608c6195ba3fe
Reviewed-on: https://chromium-review.googlesource.com/646809
Commit-Ready: Ted Lai <tedlai@chromium.org>
Tested-by: Ted Lai <tedlai@chromium.org>
Reviewed-by: Chung-yih Wang <cywang@google.com>

[modify] https://crrev.com/05fe230e4d13d027842ee0f71fb4143dc282f420/client/common_lib/cros/arc.py

The following revision refers to this bug:
  https://chromium.googlesource.com/chromiumos/third_party/autotest/+/490563bfc6028350deee19123bba18da2a85498f

commit 490563bfc6028350deee19123bba18da2a85498f
Author: Ted Lai <tedlai@google.com>
Date: Tue Sep 19 07:31:54 2017

Remove the defunct arguments email_id and password in arc_setup().

Also, expose the arguments username and password in initialize() for
clarity.

Verified that the defunct arguments are not used anywhere by this
command:
$ grep -rnw
'/usr/local/google/home/tedlai/chromiumos/src/third_party/' -e
'email_id='

BUG= chromium:740423 
TEST=test_that <DUT_IP> cheets_AuthPerf, cheets_MailBench,
cheets_PlayStoreOptIn, cheets_PlayStoreTest.

Reviewed-on: https://chromium-review.googlesource.com/646809
Commit-Ready: Ted Lai <tedlai@chromium.org>
Tested-by: Ted Lai <tedlai@chromium.org>
Reviewed-by: Chung-yih Wang <cywang@google.com>
(cherry picked from commit 05fe230e4d13d027842ee0f71fb4143dc282f420)
Change-Id: I785ba21020b86acd94e2f4c9194608c6195ba3fe
Reviewed-on: https://chromium-review.googlesource.com/672104
Commit-Queue: Ted Lai <tedlai@chromium.org>

[modify] https://crrev.com/490563bfc6028350deee19123bba18da2a85498f/client/common_lib/cros/arc.py