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As part of committed interstitials, the MergeSessionLoadPage interstitial was
removed, the wait for MergeSession now happens directly in the
MergeSessionNavigationThrottle.

Bug: 854628
Cq-Include-Trybots: luci.chromium.try:closure_compilation
Change-Id: Ib5c73824d08de8733b9b47e113587a552aa9a828
Reviewed-on: https://chromium-review.googlesource.com/1108186Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Commit-Queue: Carlos IL <carlosil@chromium.org>
Cr-Commit-Position: refs/heads/master@{#569682}