Update test_runner.py to use page_set names rather than paths.

Update test_runner.py to use page_set names rather than paths.

This allows us to get rid of the page_set dependency in discover.py,
since it eliminates the last call to IsPageSetFile.

NOTE: reverting this patch will require reverting https://codereview.chromium.org/555843003/ if that patch is landed.
BUG=

Committed: https://crrev.com/70712e0cd3f8d7d6c0b32f705a8f43bbeaae0374
Cr-Commit-Position: refs/heads/master@{#293973}

[ariblue, 2014-09-08 23:55:10]

ariblue@google.com changed reviewers:
+ nednguyen@google.com

[nednguyen, 2014-09-09 02:20:39]

lgtm
Thanks Ari! I think we can remove the page_set.PageSet.FromFile since this
should be the only usage of it too.

[nednguyen, 2014-09-09 17:05:11]

The CQ bit was checked by nednguyen@google.com

[commit-bot: I haz the power, 2014-09-09 17:13:37]

CQ is trying da patch. Follow status at
 https://chromium-status.appspot.com/cq/ariblue@google.com/551883003/20001

[commit-bot: I haz the power, 2014-09-09 18:39:13]

Committed patchset #2 (id:20001) as be16bf7ec01d2c1dae65bd9549918a3422235b7e

[commit-bot: I haz the power, 2014-09-10 03:54:27]

Patchset 2 (id:??) landed as
https://crrev.com/70712e0cd3f8d7d6c0b32f705a8f43bbeaae0374
Cr-Commit-Position: refs/heads/master@{#293973}