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During layout tests when preferences were being overriden they were not
being properly being used as the cached WebPreferences on the
RenderViewHost was not updated. This change fixes that and changes the way
spatial navigation is enabled in layout tests to adapt to this change.

This is an initial change to support the moving of mixed content checks to
the browser.

BUG=576270

Review-Url: https://codereview.chromium.org/2042763002
Cr-Commit-Position: refs/heads/master@{#398276}