CL in review on https://crrev.com/c/951586. My next working day is in a week, so I expect landing this next week. After landing and baking in Canary I'll request merging to M66.

The positive impact of this change is negligible (very few users will ever see the progress bar, and only a fraction of them will hit the bar being shown for <1 second). OTOH, there is a certain risk to all users of the export feature, because the CL references in #2 is surprisingly complicated.

Therefore I don't think it is a good idea to merge this to M66, and plan to only land it on trunk (M67). I informed maxwalker@ (Cc for the record) who is fine fine with this.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/178a7ae0bc121cdb286a7e46b002706785abc51b

commit 178a7ae0bc121cdb286a7e46b002706785abc51b
Author: Vaclav Brozek <vabr@chromium.org>
Date: Wed Mar 14 17:26:39 2018

[Android settings] Progress bar stays up for >=1s

During exporting passwords, a progressbar is shown. The UI
requirements (go/lcfve) are: once shown, the progress bar needs to
stay up for at least one second to avoid flickering. This CL
implements the artificial delay.

Bug:  817370 
Change-Id: I60729b86b1a684d6031dda2863842b0e056bce7a
Reviewed-on: https://chromium-review.googlesource.com/951586
Commit-Queue: Vaclav Brozek <vabr@chromium.org>
Reviewed-by: Bernhard Bauer <bauerb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#543117}
[add] https://crrev.com/178a7ae0bc121cdb286a7e46b002706785abc51b/chrome/android/java/src/org/chromium/chrome/browser/preferences/password/CallbackDelayer.java
[add] https://crrev.com/178a7ae0bc121cdb286a7e46b002706785abc51b/chrome/android/java/src/org/chromium/chrome/browser/preferences/password/DialogManager.java
[add] https://crrev.com/178a7ae0bc121cdb286a7e46b002706785abc51b/chrome/android/java/src/org/chromium/chrome/browser/preferences/password/ManualCallbackDelayer.java
[modify] https://crrev.com/178a7ae0bc121cdb286a7e46b002706785abc51b/chrome/android/java/src/org/chromium/chrome/browser/preferences/password/SavePasswordsPreferences.java
[add] https://crrev.com/178a7ae0bc121cdb286a7e46b002706785abc51b/chrome/android/java/src/org/chromium/chrome/browser/preferences/password/SingleThreadBarrierClosure.java
[add] https://crrev.com/178a7ae0bc121cdb286a7e46b002706785abc51b/chrome/android/java/src/org/chromium/chrome/browser/preferences/password/TimedCallbackDelayer.java
[modify] https://crrev.com/178a7ae0bc121cdb286a7e46b002706785abc51b/chrome/android/java_sources.gni
[modify] https://crrev.com/178a7ae0bc121cdb286a7e46b002706785abc51b/chrome/android/javatests/src/org/chromium/chrome/browser/preferences/password/SavePasswordsPreferencesTest.java
[add] https://crrev.com/178a7ae0bc121cdb286a7e46b002706785abc51b/chrome/android/junit/src/org/chromium/chrome/browser/preferences/password/DialogManagerTest.java
[add] https://crrev.com/178a7ae0bc121cdb286a7e46b002706785abc51b/chrome/android/junit/src/org/chromium/chrome/browser/preferences/password/EnsureAsyncPostingRule.java
[add] https://crrev.com/178a7ae0bc121cdb286a7e46b002706785abc51b/chrome/android/junit/src/org/chromium/chrome/browser/preferences/password/SingleThreadBarrierClosureTest.java
[add] https://crrev.com/178a7ae0bc121cdb286a7e46b002706785abc51b/chrome/android/junit/src/org/chromium/chrome/browser/preferences/password/TimedCallbackDelayerTest.java

The goal of this bug is achieved in r543117 (above).
I'm keeping this open until I also land UMA to measure how often this addition has been helpful.
Because it added a lot of code complexity, I want to check that it has actually been helpful. If I see it consistently unused in a few releases, I will make case with the UI review for removing it again.

Actually, let's track the metrics separately, in  bug 823695 , and mark this as done.