set webrtc handling policy in the browser process

[bridiver]

Submitter Checklist:

@diracdeltas what is a good way to test this?

• Submitted a ticket for my issue if one did not already exist.
• Used Github auto-closing keywords in the commit message.
• Added/updated tests for this change (for new code or code which already has tests).
• Ran git rebase -i to squash commits (if needed).
• Tagged reviewers and labelled the pull request as needed.

Test Plan:

Reviewer Checklist:

Tests

• Adequate test coverage exists to prevent regressions
• Tests should be independent and work correctly when run individually or as a suite ref
• New files have MPL2 license header

[diracdeltas]

@bridiver i tested this originally by navigating between https://diafygi.github.io/webrtc-ips/ and https://browserleaks.com/webrtc (having shields down for one of them). it's also covered in the fingerprinting tests.

[NejcZdovc]

@bridiver what is the status of this PR? is it ready for a review?

[bridiver]

yea, just need to verify tests

[bsclifton]

@bridiver if we'd like manual QA testing for this, can you create an issue?

It sounds like the steps @diracdeltas mentioned can be used in the test plan

navigating between https://diafygi.github.io/webrtc-ips/ and https://browserleaks.com/webrtc (having shields down for one of them)

[bsclifton]

Because it relies on previous commits (specifically, #9154), this can't go into 0.17.x. Updating milestone to be 0.18.x