Update histograms.xml with new ProxyPrefMigrationResult enum value.

Update histograms.xml with new ProxyPrefMigrationResult enum value.

A new enum value for the DataReductionProxy.ProxyPrefMigrationResult
histogram was added in https://codereview.chromium.org/1148793008, but
this new value wasn't added to histograms.xml. This CL updates
histograms.xml appropriately.

BUG=498429
Committed: https://crrev.com/8f81c5d9936bfec63253f11d6c53f59a266ef427
Cr-Commit-Position: refs/heads/master@{#333585}

[sclittle, 2015-06-09 20:23:50]

sclittle@chromium.org changed reviewers:
+ asvitkine@chromium.org

[Alexei Svitkine (slow), 2015-06-09 20:48:18]

lgtm

[sclittle, 2015-06-09 20:48:41]

The CQ bit was checked by sclittle@chromium.org

[commit-bot: I haz the power, 2015-06-09 20:48:50]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1164423009/1

[commit-bot: I haz the power, 2015-06-09 22:09:49]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-06-09 22:10:45]

Patchset 1 (id:??) landed as
https://crrev.com/8f81c5d9936bfec63253f11d6c53f59a266ef427
Cr-Commit-Position: refs/heads/master@{#333585}