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}
    virtual void SetUp()
    {
        blink::Platform::initialize(&m_proxyPlatform);

        m_globalMemoryCache = replaceMemoryCacheForTesting(MemoryCache::create());

        m_fetcher = ResourceFetcher::create(MockFetchContext::create());
    }

    virtual void SetUp()
    {
        blink::Platform::initialize(&m_proxyPlatform);

        m_globalMemoryCache = replaceMemoryCacheForTesting(MemoryCache::create());

        m_fetcher = ResourceFetcher::create(MockFetchContext::create());
    }
