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The scroll restoration type is added to page state and serialized. It is used
by blink to control the scroll/scale restoration behaviour for history
entries. Its value may be provided by user via web-exposed API.

Blink side CL: http://crrev.com/927213004

BUG=444094

Review URL: https://codereview.chromium.org/959783003

Cr-Commit-Position: refs/heads/master@{#329632}