Revert of Revert of [Telemetry] Fix WPR files not being downloaded when required.

Revert of Revert of [Telemetry] Fix WPR files not being downloaded when required. (patchset #1 id:1 of https://codereview.chromium.org/852863003/)

Reason for revert:
Reverting my revert because it didn't fix the flake! Churn, churn, churn...

Original issue's description:
> Revert of [Telemetry] Fix WPR files not being downloaded when required. (patchset #4 id:60001 of https://codereview.chromium.org/834173006/)
>
> Reason for revert:
> Possible suspect for newly introduced telemetry unit test flake, e.g.:
>
> https://build.chromium.org/p/chromium.mac/builders/Mac10.9%20Tests%20%28dbg%29/builds/1965/steps/telemetry_unittests/logs/stdio
>
> Reverting to see what happens.
>
> Original issue's description:
> > [Telemetry] Fix WPR files not being downloaded when required.
> >
> > BUG=
> >
> > Committed: https://crrev.com/672e7140315568e9bfc442770a23fee66e045cb2
> > Cr-Commit-Position: refs/heads/master@{#311496}
>
> TBR=nednguyen@google.com,aiolos@chromium.org,lizeb@chromium.org
> NOTREECHECKS=true
> NOTRY=true
> BUG=
>
> Committed: https://crrev.com/6041d4206970522a18f49afb1ba069651dc22ba2
> Cr-Commit-Position: refs/heads/master@{#311551}

TBR=nednguyen@google.com,aiolos@chromium.org,lizeb@chromium.org
NOTREECHECKS=true
NOTRY=true
BUG=

Committed: https://crrev.com/56d974ff8e82fd4a8e277e7bc167c33fd6d795a7
Cr-Commit-Position: refs/heads/master@{#311588}

[Ken Rockot(use gerrit already), 2015-01-15 00:34:49]

Created Revert of Revert of [Telemetry] Fix WPR files not being downloaded when
required.

[commit-bot: I haz the power, 2015-01-15 00:35:36]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/794363005/1

[commit-bot: I haz the power, 2015-01-15 00:36:43]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-01-15 00:37:27]

Patchset 1 (id:??) landed as
https://crrev.com/56d974ff8e82fd4a8e277e7bc167c33fd6d795a7
Cr-Commit-Position: refs/heads/master@{#311588}