Use base::IsNaN in page_state_serialization_unittest.cc

Using plain 'isnan' breaks on certain compilers if 'cmath' is included
via any dependency trail, and base::IsNaN already handles that and other
platforms (e.g. Windows).

BUG=

Review URL: https://codereview.chromium.org/1054543003

Cr-Commit-Position: refs/heads/master@{#323594}
1 file changed