Add assertions that the empty Platform::cryptographicallyRandomValues() overrides are not being used.

These implementations are not safe and look scary if not accompanied by an assertion. Also one of the comments was incorrect.

BUG=552749

Review URL: https://codereview.chromium.org/1419293005

Cr-Commit-Position: refs/heads/master@{#359229}
        ~ProxyPlatform()
        {
            blink::Platform::initialize(m_platform);
        }

        ~ProxyPlatform()
        {
            blink::Platform::initialize(m_platform);
        }
