Move unique name generation and tracking into //content.

Blink doesn't really consume this anymore, so move the
concept out of Blink and simplify the plumbing. Future
CLs will move it wholly into the browser process.

BUG=626202,645123
CQ_INCLUDE_TRYBOTS=master.tryserver.chromium.linux:linux_site_isolation
[email protected], [email protected], [email protected]

Review-Url: https://codereview.chromium.org/2714943004 .
Cr-Commit-Position: refs/heads/master@{#458633}
49 files changed