protocol::HashMap(std::unordered_map)::iterator doesn't provide any
guarantees about iteration order. At least Visual C++ from vS 2015 and
clang compile for loop inside forEachSession differently.
For tests we need stable order of iteration, so let's use std::map
instead.

R=dgozman@chromium.org

Bug: none
Cq-Include-Trybots: master.tryserver.blink:linux_trusty_blink_rel
Change-Id: I329cb24cd182baa86c0ea4a526257856718f32b1
Reviewed-on: https://chromium-review.googlesource.com/609489Reviewed-by: Dmitry Gozman <dgozman@chromium.org>
Commit-Queue: Aleksey Kozyatinskiy <kozyatinskiy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#47281}