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}
 const unsigned char* TestingPlatformSupport::getTraceCategoryEnabledFlag(const char* categoryName)
{
    static const unsigned char tracingIsDisabled = 0;
    return &tracingIsDisabled;
}

 const unsigned char* TestingPlatformSupport::getTraceCategoryEnabledFlag(const char* categoryName)
{
    static const unsigned char tracingIsDisabled = 0;
    return &tracingIsDisabled;
}
