Add securityPropertyUnknown to devtools.gypi
Add securityPropertyUnknown to devtools.gypi

BUG=549410
TEST=Open 'https://mixed.badssl.com/test/canceled-image/' in a tab, open DevTools on the tab and activate Security Panel, refresh source tab to reveal origins, observe that the google.com origin has a '?' icon as its bullet.

Review URL: https://codereview.chromium.org/1439493002

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#359147}

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