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TabModel is provided to the CrashRestoreHelper as a SessionWindowRestore
protocol delegate. and it's used to restore sessions from there. Replace
that with browser and use sessionRestorationBrowserAgent from the
CrashRestoreHelper instead.
Also while at it, i changed the members ivars to use trailing underscore
instead of the leading underscore.

Bug: 1010164
Change-Id: I53360ac9a34f7f9e42aacd459e233f40c5def60b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2024028
Commit-Queue: Mohammad Refaat <mrefaat@chromium.org>
Reviewed-by: Mark Cogan <marq@chromium.org>
Cr-Commit-Position: refs/heads/master@{#736047}