Send timestamps in quic logging as a string rather than "int".
Send timestamps in quic logging as a string rather than "int".

This works around truncation problems in casting int64 to an int.

This does change the format of net-log slightly (what was once numbers are now strings), but nothing seems to be relying on this.

BUG=496127

Review URL: https://codereview.chromium.org/1161593003

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#332836}

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