*Don't* disable CacheManager when using Chromium HTTP stack.

This reverts CL https://android-git.corp.google.com/g/84508

After lengthy discussion we have decided that disabling CacheManager
is too risky for third-party apps, and that enough functionality is
still available for it to be useful.

WebView.loadUrl() will not add pages to the cache, but it is still
possible to add them manually via CacheManager.saveCacheFile().

Change-Id: I6cfd9f93bb8d685450f048375adad9d17fa4eeba