Introducing a net::GenerateMimeMultipartBoundary helper.

... and using the helper in places that had introduced separate
functions for generating a random Mime boundary (sometimes with
bugs - i.e. using '*' characters is disallowed by RFC 1341).

[email protected], [email protected]
BUG=575733

Review URL: https://codereview.chromium.org/1547593002

Cr-Commit-Position: refs/heads/master@{#368404}
18 files changed