Looks like a bug. We always install the watcher even though the file system is not watchable. For now you can just ignore those calls.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/df904a379b5050fca5d641871797f57c8c2f9e60

commit df904a379b5050fca5d641871797f57c8c2f9e60
Author: Tomasz Mikolajewski <mtomasz@chromium.org>
Date: Wed Jan 10 03:38:04 2018

Do not watch not watchable file systems.

Provided file systems which are not watchable shouldn't have it's
*Watcher methods called.

Test: Browser tests.
Bug:  796334 
Change-Id: I758a922359c64b5fcc2b9ba2dd4b8e5d0c3ed9c6
Reviewed-on: https://chromium-review.googlesource.com/839500
Reviewed-by: Naoki Fukino <fukino@chromium.org>
Commit-Queue: Tomasz Mikolajewski <mtomasz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#528230}
[modify] https://crrev.com/df904a379b5050fca5d641871797f57c8c2f9e60/chrome/browser/chromeos/file_system_provider/fileapi/watcher_manager.cc
[modify] https://crrev.com/df904a379b5050fca5d641871797f57c8c2f9e60/chrome/test/data/extensions/api_test/file_system_provider/add_watcher/manifest.json
[modify] https://crrev.com/df904a379b5050fca5d641871797f57c8c2f9e60/chrome/test/data/extensions/api_test/file_system_provider/notify/manifest.json
[modify] https://crrev.com/df904a379b5050fca5d641871797f57c8c2f9e60/chrome/test/data/extensions/api_test/file_system_provider/remove_watcher/manifest.json

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/072d519850dea414398c1e3dcea4c1ed7b21fa6e

commit 072d519850dea414398c1e3dcea4c1ed7b21fa6e
Author: Bailey Berro <baileyberro@chromium.org>
Date: Fri Mar 23 17:07:48 2018

clean up SmbFileSystem member variables

This change includes some cleanup, removing unused member variables and
removing a reference to a bug that has been fixed.

Bug:  chromium:757625 ,  chromium:796334 
Change-Id: I6a5901f355050a47aeb59850e34ed463b79b6899
Reviewed-on: https://chromium-review.googlesource.com/978379
Reviewed-by: Zentaro Kavanagh <zentaro@chromium.org>
Commit-Queue: Zentaro Kavanagh <zentaro@chromium.org>
Cr-Commit-Position: refs/heads/master@{#545494}
[modify] https://crrev.com/072d519850dea414398c1e3dcea4c1ed7b21fa6e/chrome/browser/chromeos/smb_client/smb_file_system.cc
[modify] https://crrev.com/072d519850dea414398c1e3dcea4c1ed7b21fa6e/chrome/browser/chromeos/smb_client/smb_file_system.h

It seems that GetWatchers is still being called in some places. Specifically: https://cs.chromium.org/chromium/src/chrome/browser/chromeos/file_system_provider/service.cc?dr=CSs&sq=package:chromium&l=185

and 

https://cs.chromium.org/chromium/src/chrome/browser/chromeos/file_system_provider/service.cc?dr=CSs&sq=package:chromium&l=379

I submitted a CL that adds a check to watchable() to address these issues.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/67af16ed4b5c8dcca09f8b6dd0d892c4e9e57c93

commit 67af16ed4b5c8dcca09f8b6dd0d892c4e9e57c93
Author: Allen Vicencio <allenvic@chromium.org>
Date: Wed Mar 28 12:25:12 2018

[FSP] Do not get watchers for unwatchable file systems

Provided file systems which are not watchable shouldn't have its
GetWatchers method called during Mount and RestoreFileSystems.

Test: Unit tests pass and manual test of RestoreFileSystems
Bug:  796334 
Change-Id: Ibc592e20f8f2cb26843117aa31ec5fcfc627f870
Reviewed-on: https://chromium-review.googlesource.com/982646
Reviewed-by: Tomasz Mikolajewski <mtomasz@chromium.org>
Reviewed-by: Naoki Fukino <fukino@chromium.org>
Reviewed-by: Noel Gordon <noel@chromium.org>
Commit-Queue: Noel Gordon <noel@chromium.org>
Cr-Commit-Position: refs/heads/master@{#546461}
[modify] https://crrev.com/67af16ed4b5c8dcca09f8b6dd0d892c4e9e57c93/chrome/browser/chromeos/file_system_provider/service.cc
[modify] https://crrev.com/67af16ed4b5c8dcca09f8b6dd0d892c4e9e57c93/chrome/browser/chromeos/file_system_provider/service_unittest.cc

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/e5c8b23f960e569acf621534dcb478eaa7c3ed62

commit e5c8b23f960e569acf621534dcb478eaa7c3ed62
Author: Bailey Berro <baileyberro@chromium.org>
Date: Wed Apr 04 17:27:10 2018

Do not call GetWatchers() on unwatchable provider

This is a follow up on crrev.com/c/982646

Bug:  796334 
Change-Id: I63b04d2f2ab61ef1d9a45f3d360df5f1e1290959
Reviewed-on: https://chromium-review.googlesource.com/988742
Reviewed-by: Naoki Fukino <fukino@chromium.org>
Reviewed-by: Zentaro Kavanagh <zentaro@chromium.org>
Commit-Queue: Bailey Berro <baileyberro@chromium.org>
Cr-Commit-Position: refs/heads/master@{#548111}
[modify] https://crrev.com/e5c8b23f960e569acf621534dcb478eaa7c3ed62/chrome/browser/chromeos/extensions/file_system_provider/file_system_provider_api.cc