Omit policy-installed certificate message on unsupported platforms

Only ChromeOS supports policy-installed certificates. Omit the string
describing this state from the PageInfo UI's resources except when
compiling for ChromeOS.

Bug: 717802
Change-Id: I106b03b8cac1a15e3937996585185e2e26279d4a
Reviewed-on: https://chromium-review.googlesource.com/779880


Reviewed-by: Emily Stark <estark@chromium.org>
Commit-Queue: Eric Lawrence <elawrence@chromium.org>
Cr-Commit-Position: refs/heads/master@{#518267}