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This CL adds a new resource type field to the manifest proto.
ResourcePrefetchPredictor begins to use this field to prefetch all resources of
one resource type before another.

Also we prioritize stylesheets above scripts since this CL. Stylesheets are
almost always critical, scripts are not. It's clear win in the case if we're
fetching resources from manifests because we don't have information about
request priority in manifests. Thus we can't separate critical scripts from
non-critical ones.

BUG=699115

Review-Url: https://codereview.chromium.org/2831233004
Cr-Commit-Position: refs/heads/master@{#467742}