predictors: Refactor the resource_prefetch_predictor database handling.

predictors: Refactor the resource_prefetch_predictor database handling.

This is a follow-up to crrev.com/2263243002. The ResourceRow object is
no longer needed, this removes it.

BUG=631966

[Benoit L, 2016-08-24 16:02:05]

The CQ bit was checked by lizeb@chromium.org to run a CQ dry run

[commit-bot: I haz the power, 2016-08-24 16:02:40]

Dry run: CQ is trying da patch. Follow status at
 
https://chromium-cq-status.appspot.com/v2/patch-status/codereview.chromium.or...

[commit-bot: I haz the power, 2016-08-24 17:30:49]

The CQ bit was unchecked by commit-bot@chromium.org

[commit-bot: I haz the power, 2016-08-24 17:30:50]

Dry run: Try jobs failed on following builders:
  linux_android_rel_ng on master.tryserver.chromium.android (JOB_FAILED,
https://build.chromium.org/p/tryserver.chromium.android/builders/linux_androi...)

[Benoit L, 2016-08-25 11:47:39]

lizeb@chromium.org changed reviewers:
+ pasko@chromium.org

[pasko, 2016-08-25 12:33:09]

Discussed offline, a few topics to figure out:

1. do we really need a separate column for url (if not, can handle it in
ResourceData)

2. the formula for score can change, so we should compute it on every read
anyway, then no reason to persist it to disk

3. inheriting from ResourceData and adding methods to backfill score and primary
key would make for a more encapsulated datastructure. Composition is a good
alternative. Worth it?