The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/12350097b18cf5a7225a57138834ce9d62b741af

commit 12350097b18cf5a7225a57138834ce9d62b741af
Author: janeliulwq <janeliulwq@google.com>
Date: Wed Aug 24 16:49:28 2016

[MD User Menu] Adjusted its background color

See first bug.
BUG= 636073 
BUG= 615893 

Review-Url: https://codereview.chromium.org/2241293003
Cr-Commit-Position: refs/heads/master@{#414092}

[modify] https://crrev.com/12350097b18cf5a7225a57138834ce9d62b741af/ui/native_theme/native_theme_mac.mm
[modify] https://crrev.com/12350097b18cf5a7225a57138834ce9d62b741af/ui/native_theme/native_theme_win.cc

Discussion with estade@ led to the conclusion that the bkg color for all the dialogs should be #FFFFFF when #secondary-ui-md (the flag for extending MD settings to secondary UIs like dialogs, bubbles, etc, which includes the user menu) is turned on, which is achieved by the above CL.
In other words, on Windows and Mac, bkg color would be #FFFFFF when #secondary-ui-md is turned on; on Linux, bkg color is already #FFFFFF.

OK. Alan - does that make sense to you?

Just an FYI - according to estade@, #secondary-ui-md is targeted to be turned on in M55.

That makes sense. SGTM!