Remove LOAD_SUB_FRAME load flag.

It's not used in net, and net/ knowing about frames is a layering
violation.  External consumers that depended on it now use the
corresponding ResourceType instead.

BUG=426442

Review URL: https://codereview.chromium.org/1134733004

Cr-Commit-Position: refs/heads/master@{#330370}
7 files changed