[Telemetry] Change the page's file_path param to base_dir param

[Telemetry]  Change the page's file_path param to base_dir param.

This help the conversion from page_set to story_set easier.

BUG=439512
CQ_EXTRA_TRYBOTS=tryserver.chromium.perf:mac_perf_bisect;tryserver.chromium.perf:win_perf_bisect;tryserver.chromium.perf:android_nexus5_perf_bisect

Committed: https://crrev.com/0cdbcd5d6e5a05aa069d865ecd5fb83d3012fe80
Cr-Commit-Position: refs/heads/master@{#333774}

[nednguyen, 2015-06-10 02:16:01]

nednguyen@google.com changed reviewers:
+ aiolos@chromium.org, kbr@chromium.org

[nednguyen, 2015-06-10 03:10:43]

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

[nednguyen, 2015-06-10 03:15:18]

Patchset #1 (id:1) has been deleted

[nednguyen, 2015-06-10 14:56:57]

nednguyen@google.com changed reviewers:
+ eakuefner@chromium.org

[Ken Russell (switch to Gerrit), 2015-06-10 16:45:34]

content/test/gpu lgtm

[aiolos (Not reviewing), 2015-06-10 17:01:57]

lgtm.

I find it odd that gpu uses lowercase for global variables (including the
dir_path), but it seems to be consistent across the directory. :-/

[eakuefner, 2015-06-10 17:04:43]

lgtm nice work

[nednguyen, 2015-06-10 17:23:12]

The CQ bit was checked by nednguyen@google.com

[commit-bot: I haz the power, 2015-06-10 17:23:52]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1176733003/20001

[commit-bot: I haz the power, 2015-06-10 19:24:39]

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

[commit-bot: I haz the power, 2015-06-10 19:24:40]

Try jobs failed on following builders:
  linux_perf_bisect on tryserver.chromium.perf (JOB_TIMED_OUT, no build URL)

[nednguyen, 2015-06-10 19:31:18]

The CQ bit was checked by nednguyen@google.com

[commit-bot: I haz the power, 2015-06-10 19:31:40]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1176733003/20001

[commit-bot: I haz the power, 2015-06-10 19:37:44]

Committed patchset #1 (id:20001)

[commit-bot: I haz the power, 2015-06-10 19:38:45]

Patchset 1 (id:??) landed as
https://crrev.com/0cdbcd5d6e5a05aa069d865ecd5fb83d3012fe80
Cr-Commit-Position: refs/heads/master@{#333774}