Add UMA to understand the typical total size of ServiceWorker script [2/2 chromium]

Add UMA to understand the typical total size of ServiceWorker script [2/2 chromium]

This cl introduces three UMAs.
- ServiceWorker.ScriptCount
- ServiceWorker.ScriptTotalSize
- ServiceWorker.ScriptCachedMetadataTotalSize

1/2 blink: https://codereview.chromium.org/1059193002/
2/2 chromium: This cl.

BUG=408451
Committed: https://crrev.com/1085f573094f9d09978915bef54a5283c20782b2
Cr-Commit-Position: refs/heads/master@{#324180}

[horo, 2015-04-03 08:37:16]

Patchset #1 (id:1) has been deleted

[horo, 2015-04-03 08:37:42]

horo@chromium.org changed reviewers:
+ ksakamoto@chromium.org

[horo, 2015-04-03 08:37:42]

ksakamoto@
Could you please review this?

[Kunihiko Sakamoto, 2015-04-03 09:10:08]

lgtm

[horo, 2015-04-03 09:13:55]

horo@chromium.org changed reviewers:
+ jwd@chromium.org

[horo, 2015-04-03 09:13:56]

jwd@
Could you please review this?

[jwd, 2015-04-07 15:33:00]

lgtm

[horo, 2015-04-08 01:08:48]

The CQ bit was checked by horo@chromium.org

[commit-bot: I haz the power, 2015-04-08 01:09:03]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1055423002/20001

[commit-bot: I haz the power, 2015-04-08 02:51:24]

Committed patchset #1 (id:20001)

[commit-bot: I haz the power, 2015-04-08 02:52:18]

Patchset 1 (id:??) landed as
https://crrev.com/1085f573094f9d09978915bef54a5283c20782b2
Cr-Commit-Position: refs/heads/master@{#324180}