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On WebVR pages loaded over insecure transport, show a small "Not secure"
warning overlay permanently, and a transient more verbose warning for
a short time on first entering the page.

It uses the information from bajones's "Expose secure origin state to
WebVR renderer" implementation.

The message strings were previously added to chrome/app/generated_resources.grd in https://codereview.chromium.org/2368443002/

BUG=389343,644492
CQ_INCLUDE_TRYBOTS=master.tryserver.chromium.linux:closure_compilation

Review-Url: https://codereview.chromium.org/2363553003
Cr-Commit-Position: refs/heads/master@{#422647}