Change the type of session_id() from string to int64_t.

BUG=webrtc:7311

Review-Url: https://codereview.webrtc.org/2749493002
Cr-Original-Commit-Position: refs/heads/master@{#17215}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: 30e0da4a65d4ded14a9158b61548ce09d7cc95c1
2 files changed