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Child frames have some degree of continuity across navigations: the
contents are still embedded in the same document. Let them continue to
propagate user gesture state across navigations. However, main frames
don't have that continuity and should reset their user gesture state
on navigation.

TEST=DocumentUserGestureTokenTest.Navigate
BUG=chromedriver:922

Review-Url: https://codereview.chromium.org/2721423003
Cr-Commit-Position: refs/heads/master@{#456134}