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Rationale:
+ This makes _RunPage --> SharedPageState refactoring easier.
+ This enables combining pages from different page_sets.

This patch also modifies page_set_smoke_test.py to make sure that the change doesn't break existing page sets.

BUG=418278

Review URL: https://codereview.chromium.org/625123002

Cr-Commit-Position: refs/heads/master@{#298796}