Set minimum_zoom_scale for PageScrollState tests.
Set minimum_zoom_scale for PageScrollState tests.

For WKWebView on iOS9 minimum_zoom_scale is 0.25, but on
UIWebView and WKWebView pre-iOS9, it was set to 1. This change
explicitly sets the minimum_zoom_scale for the tests.

BUG=517936

Review URL: https://codereview.chromium.org/1277223003

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#342726}

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