core/loader: create private directory to store private classes

To store classes that should not be used outside core/loader, create
private directory and store such files under the directory.

This is not a well-known convention in chromium or blink, and does not
enforce it unlike DEPS rule.

BUG=692998

Review-Url: https://codereview.chromium.org/2700073002
Cr-Commit-Position: refs/heads/master@{#451577}
11 files changed