Dont preload from link headers if the response came from MemoryCache

This patch ensures that in we don't end up in a stack overflow if a
resource in memory cache has link headers directing to preload itself.

BUG=628904

Review-Url: https://codereview.chromium.org/2160403002
Cr-Commit-Position: refs/heads/master@{#406534}
3 files changed