page_load_metrics symbolic constants refactor

page_load_metrics symbolic constants refactor
We had const string constants for histogram names used for tests, but
we should unify the code and use them everywhere. This change also
corrects the names to be more standard (i.e. BG->Background).

BUG=549295
Committed: https://crrev.com/980920bd4915931d6871cbfe62d717a9956947cb
Cr-Commit-Position: refs/heads/master@{#359781}

[Charlie Harrison, 2015-11-06 18:09:19]

csharrison@chromium.org changed reviewers:
+ bmcquade@chromium.org, rdsmith@chromium.org

[Charlie Harrison, 2015-11-06 18:09:19]

PTAL. cc nasko@ who brought up this issue with me.

[nasko, 2015-11-06 23:33:25]

On 2015/11/06 18:09:19, csharrison wrote:
> PTAL. cc nasko@ who brought up this issue with me.

Thanks for following up on this!

[Bryan McQuade, 2015-11-07 16:31:14]

lgtm

[Randy Smith (Not in Mondays), 2015-11-09 20:24:09]

Structure LGTM.  I did not verify that each change was correct (i.e. that you
used the correct constant for each previous string or differently named
constant); let me know if you want me to go through and do that.  (I'm happy to
stamp based on your judgement on that.)

[Charlie Harrison, 2015-11-09 21:33:00]

The CQ bit was checked by csharrison@chromium.org

[Charlie Harrison, 2015-11-09 21:33:31]

The CQ bit was unchecked by csharrison@chromium.org

[Charlie Harrison, 2015-11-09 21:34:38]

It's okay, I just self reviewed for errors.

[Charlie Harrison, 2015-11-09 21:34:39]

It's okay, I just self reviewed for errors.

[Charlie Harrison, 2015-11-15 19:38:18]

The CQ bit was checked by csharrison@chromium.org

[Charlie Harrison, 2015-11-15 19:38:19]

The patchset sent to the CQ was uploaded after l-g-t-m from
bmcquade@chromium.org, rdsmith@chromium.org
Link to the patchset: https://codereview.chromium.org/1417733011/#ps40001
(title: " ")

[commit-bot: I haz the power, 2015-11-15 19:38:29]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1417733011/40001
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1417733011/40001

[commit-bot: I haz the power, 2015-11-15 19:41:44]

Committed patchset #3 (id:40001)

[commit-bot: I haz the power, 2015-11-15 19:43:34]

Patchset 3 (id:??) landed as
https://crrev.com/980920bd4915931d6871cbfe62d717a9956947cb
Cr-Commit-Position: refs/heads/master@{#359781}