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On user session start, only request the chrome process to be restarted
if site isolation policies effectively change between the sign-in screen
and the user session.
The DeviceLoginScreenSitePerProcess and DeviceLoginScreenIsolateOrigins
policies map to --site-per-process and --isolate-origins command-line
flags passed to the sign-in screen chrome process by session manager
(see CL:924421).
The user session should respect SitePerProcess/IsolateOrigins user
policies instead.

Replace special handling of the site isolation flags with placing them
between --policy-flags-begin and --policy-flags-end sentinels.
This makes sure that chrome is restarted exactly when the user session
flags don't match the login screen flags.
Background: chrome decides if it should be restarted by comparing the
parts of the current command line with parts of the target command line.
Specifically, the parts between flags and policy sentinels are compared.

Also, evaluate the user policies mapped through user Profile prefs
instead of local_state.
Reason: When login_manager starts chrome with site isolation flags, the
prefs are set by ChromeCommandLinePrefStore.

TBR=xiyuan@chromium.org

Bug: 800117
Test: browser_tests --gtest_filter=*SiteIsolationFlagHandlingTest*
Change-Id: I85d47bd2a4da9155e9c6ad338284e1bfed5b478e
Reviewed-on: https://chromium-review.googlesource.com/924147
Commit-Queue: Pavol Marko <pmarko@chromium.org>
Reviewed-by: Maksim Ivanov <emaxx@chromium.org>
Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Cr-Original-Commit-Position: refs/heads/master@{#537733}
Reviewed-on: https://chromium-review.googlesource.com/926370
Cr-Commit-Position: refs/heads/master@{#537793}