CL here: https://chromium-review.googlesource.com/c/chromium/src/+/1414891

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/11b37cd3d41c2e88aa837f9e8b60230fa0d90af2

commit 11b37cd3d41c2e88aa837f9e8b60230fa0d90af2
Author: Mario Sanchez Prada <mario@igalia.com>
Date: Wed Jan 16 12:41:02 2019

Eliminate oauth2_token_service_observer_bridge*

This is no longer used for the iOS build and can be safely removed.

Bug:  921615 
Change-Id: Iccc65ed6ce63f081f0a76d9d3d59786c898cdc7d
Reviewed-on: https://chromium-review.googlesource.com/c/1414891
Reviewed-by: Jérôme Lebel <jlebel@chromium.org>
Commit-Queue: Mario Sanchez Prada <mario@igalia.com>
Cr-Commit-Position: refs/heads/master@{#623207}
[modify] https://crrev.com/11b37cd3d41c2e88aa837f9e8b60230fa0d90af2/components/signin/ios/browser/BUILD.gn
[delete] https://crrev.com/f425cfd3729d3705682c51f3cd49aa77a5507e83/components/signin/ios/browser/oauth2_token_service_observer_bridge.h
[delete] https://crrev.com/f425cfd3729d3705682c51f3cd49aa77a5507e83/components/signin/ios/browser/oauth2_token_service_observer_bridge.mm