Download Home : Dont show headers when showing prefetch tab

This CL adds the following changes:
1 - On prefetch tab, date headers and section headers are not shown.
2 - On chips other than type ALL, section headers are not shown.

Bug: 866307
Change-Id: I0f184f31a80fc1d4e5defab780f430590532f517
Reviewed-on: https://chromium-review.googlesource.com/1162517
Commit-Queue: Shakti Sahu <shaktisahu@chromium.org>
Reviewed-by: Theresa <twellington@chromium.org>
Cr-Commit-Position: refs/heads/master@{#581107}