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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=

Review URL: https://codereview.chromium.org/1071203002

Cr-Commit-Position: refs/heads/master@{#325469}