+yfriedman as we're actively trying to kill all of that code

That being said, your investigation looks correct to me as well.  It looks like RemoveBookmarkTask and UpdateBookmarkTask are the two things that would need to be updated.

From what I can tell, it seems like we could get rid of the Add/Remove observer in the base class.  I think we could just add and remove the observer at the beginning of their corresponding RunOnUIThread calls.  Right now, they assume the observers are synchronous so that should work based on what I can tell.

That being said, this is old crufty code and I don't recall if Yaron has any better knowledge of this than I do.

Ya, sadly the uma metrics show that chromebrowserprovider is still fairly used on pre-m devices :(

https://uma.googleplex.com/p/chrome/histograms?endDate=20180502&dayCount=7&histograms=Android.ChromeBrowserProvider.CallerHasPermission.READ_HISTORY_BOOKMARKS%2CAndroid.ChromeBrowserProvider.CallerHasPermission.WRITE_HISTORY_BOOKMARKS%2CAndroid.ChromeBrowserProvider.SignaturePassed.READ_HISTORY_BOOKMARKS%2CAndroid.ChromeBrowserProvider.SignaturePassed.WRITE_HISTORY_BOOKMARKS&fixupData=true&showMax=true&filters=platform%2Ceq%2CA%2Cchannel%2Ceq%2C4%2Cisofficial%2Ceq%2CTrue&implicitFilters=isofficial

If we look at M+ it's a lot less:
https://uma.googleplex.com/p/chrome/histograms?endDate=20180502&dayCount=7&histograms=Android.ChromeBrowserProvider.CallerHasPermission.READ_HISTORY_BOOKMARKS%2CAndroid.ChromeBrowserProvider.CallerHasPermission.WRITE_HISTORY_BOOKMARKS%2CAndroid.ChromeBrowserProvider.SignaturePassed.READ_HISTORY_BOOKMARKS%2CAndroid.ChromeBrowserProvider.SignaturePassed.WRITE_HISTORY_BOOKMARKS&fixupData=true&showMax=true&filters=platform%2Ceq%2CA%2Cchannel%2Ceq%2C4%2Cosflavor%2Cone_of%2CAndroid_Marshmallow%7CAndroid_N%7CAndroid_O%7CAndroid_P%2Cisofficial%2Ceq%2CTrue&implicitFilters=isofficial

But we'd have to knowingly break some users

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/f5e25550afc8a4ab53915764b7a8edfe0684329f

commit f5e25550afc8a4ab53915764b7a8edfe0684329f
Author: Ted Choc <tedchoc@google.com>
Date: Fri Jun 08 17:21:26 2018

Fix ChromeBrowserProvider bookmark observer thread issue.

The BookmarkModelObserverTask added a bookmark model observer
on a background thread, which is not allowed.

In both cases of Remove and Update, there is no need for listening
to the observer as we can just track whether we are updating the
bookmark node internally.  This is consistent with how
AddBookmarkTask already worked.

BUG= 838988 

Change-Id: I43bf054b70e78907a99ba2a11fac62df0af6bc67
Reviewed-on: https://chromium-review.googlesource.com/1089497
Reviewed-by: Yaron Friedman <yfriedman@chromium.org>
Commit-Queue: Ted Choc <tedchoc@chromium.org>
Cr-Commit-Position: refs/heads/master@{#565667}
[modify] https://crrev.com/f5e25550afc8a4ab53915764b7a8edfe0684329f/chrome/browser/BUILD.gn
[delete] https://crrev.com/3af77af733bd2cd498b2aef95d080037c4929d3a/chrome/browser/android/provider/bookmark_model_observer_task.cc
[delete] https://crrev.com/3af77af733bd2cd498b2aef95d080037c4929d3a/chrome/browser/android/provider/bookmark_model_observer_task.h
[add] https://crrev.com/f5e25550afc8a4ab53915764b7a8edfe0684329f/chrome/browser/android/provider/bookmark_model_task.cc
[add] https://crrev.com/f5e25550afc8a4ab53915764b7a8edfe0684329f/chrome/browser/android/provider/bookmark_model_task.h
[modify] https://crrev.com/f5e25550afc8a4ab53915764b7a8edfe0684329f/chrome/browser/android/provider/chrome_browser_provider.cc