Consistently handle persistent notification ids as int64_t

Consistently handle persistent notification ids as int64_t

This changes the renderer side of Chromium to only denote persistent
notification ids as int64_ts rather than as strings.

This CL is part of a three-sided patch:
  [1] https://codereview.chromium.org/1078783002/
  [2] This CL.
  [3] https://codereview.chromium.org/1072873002/

BUG=

Committed: https://crrev.com/f08addc1cd8b71b8e60c6b33838ff4d02c62a9a1
Cr-Commit-Position: refs/heads/master@{#325469}

[Peter Beverloo, 2015-04-09 16:19:48]

peter@chromium.org changed reviewers:
+ johnme@chromium.org, michaeln@chromium.org, mkwst@chromium.org

[Peter Beverloo, 2015-04-09 16:19:49]

+johnme for notifications
+mkwst for IPC
+michaeln for service_worker stamp (string -> int64)

[michaeln, 2015-04-10 00:00:10]

r/s lgtm

[Mike West, 2015-04-10 11:13:10]

IPC LGTM.

[johnme, 2015-04-10 11:13:40]

notifications lgtm

[Peter Beverloo, 2015-04-16 14:36:03]

The CQ bit was checked by peter@chromium.org

[Peter Beverloo, 2015-04-16 14:36:04]

The patchset sent to the CQ was uploaded after l-g-t-m from johnme@chromium.org,
michaeln@chromium.org, mkwst@chromium.org
Link to the patchset: https://codereview.chromium.org/1071203002/#ps20001
(title: "rebase")

[commit-bot: I haz the power, 2015-04-16 14:36:28]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1071203002/20001

[commit-bot: I haz the power, 2015-04-16 17:37:51]

Committed patchset #2 (id:20001)

[commit-bot: I haz the power, 2015-04-16 17:38:36]

Patchset 2 (id:??) landed as
https://crrev.com/f08addc1cd8b71b8e60c6b33838ff4d02c62a9a1
Cr-Commit-Position: refs/heads/master@{#325469}