[Telemetry] Add 'important' to JSON output.

[Telemetry] Add 'important' to JSON output.

We have a plan in progress to upgrade the 'important' field to a combination
of two fields 'primary' and 'improvement_direction'. This will be a two-sided
patch that will require reworking the dashboard's handling of chart JSON. Since
the dashboard has been modified to accept chart JSON with 'important' values,
we want to update Telemetry to continue serializing values with this field for
now.

R=nednguyen

Committed: https://crrev.com/ba3e74deda690c0c30877a3396b4c0c9c6bbb497
Cr-Commit-Position: refs/heads/master@{#293468}

[nednguyen, 2014-09-05 03:03:11]

lgtm

[nednguyen, 2014-09-05 03:03:11]

The CQ bit was checked by nednguyen@google.com

[nednguyen, 2014-09-05 03:03:12]

lgtm

lgtm

[commit-bot: I haz the power, 2014-09-05 03:03:54]

CQ is trying da patch. Follow status at
 https://chromium-status.appspot.com/cq/eakuefner@chromium.org/541253002/1

[commit-bot: I haz the power, 2014-09-05 08:11:36]

Committed patchset #1 (id:1) as 80de52c1fd28c8d77e9e660247c88522204130bb

[commit-bot: I haz the power, 2014-09-10 03:37:45]

Patchset 1 (id:??) landed as
https://crrev.com/ba3e74deda690c0c30877a3396b4c0c9c6bbb497
Cr-Commit-Position: refs/heads/master@{#293468}