max-age without ETag nor Last-Modified should use cache without validation

[Alan Tam]

Nowadays it is a common pattern to use "?timestamp=UNIX_TIMESTAMP" in combination of "Cache-Control: max-age=INFINITY" response headers to denote scenarios where HTML or JavaScript knows that the resource we are retrieving has not been changed since UNIX_TIMESTAMP, so the browser should not attempt to revalidate the resource anyhow, including If-Modified-Since or If-None-Match requests.

Currently WebCore/loader/cache/CachedResourceLoader.cpp says:
    // Check if the cache headers requires us to revalidate (cache expiration for example).
    if (existingResource->mustRevalidateDueToCacheHeaders(cachePolicy())) {
        // See if the resource has usable ETag or Last-modified headers.
        if (existingResource->canUseCacheValidator())
            return Revalidate;
        // No, must reload.
        LOG(ResourceLoading, "CachedResourceLoader::determineRevalidationPolicy reloading due to missing cache validators.");
        return Reload;
    }

This kind of responses has canUseCacheValidator() == FALSE, so it returns Reload.
I expect it somehow returns Use, so that MemoryCache is reused as long as the resource is still fresh (i.e. max-age not yet elapsed).

[Brent Fulgham]

<rdar://problem/8917598>