Separate preaload matching from MemoryCache

This change decouples preload matching from MemoryCache lookup. With this change,
preload matching is done for ResourceFetcher::prealods_.

This is the first step and part of the second step in the design document[1].

1: https://docs.google.com/document/d/1oq8ixPnaDxuAlKUTRQ3WoYHlJenVkzNU9xnkuZX_dWM/edit#

BUG=695939

Review-Url: https://codereview.chromium.org/2834733003
Cr-Commit-Position: refs/heads/master@{#469046}
11 files changed