CSP: Implement 'prefetch-src' behind a flag.

[chromium-wpt-export-bot]

As discussed in w3c/webappsec-csp#107,
'prefetch-src' gives developers the ability to control the endpoints
from which resources may be prefetched.

Bug: 801561
Change-Id: Ifedd78e3101ea66d242c6f3c7a2f49385a681bd1
Reviewed-on: https://chromium-review.googlesource.com/864362
Commit-Queue: Mike West [email protected]
Reviewed-by: Yoav Weiss [email protected]
Cr-Commit-Position: refs/heads/master@{#529413}

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This change is 

[wpt-pr-bot]

Already reviewed downstream.

[ghost]

Build PASSED

Started: 2018-01-17 10:06:54
Finished: 2018-01-17 10:30:14

View more information about this build on:

• WPT PR Status
• TravisCI

[yoavweiss]

LGTM

[Hexcles]

@mikewest seems flaky on Firefox. 1/10 timeout, same after retry.

[mikewest]

@yoavweiss: Do you know who at Mozilla might be able to give feedback on their prefetch implementation and what I could do to make it less flaky? :)

[yoavweiss]

I'm not sure, but maybe @ddragana would know

[mikewest]

The new test implementation looks more stable. Yay!

[Hexcles]

Looks like a flake in the Travis infra blocked the merge. Restarted Travis and have my fingers crossed.

[mikewest]

Hrm. The Travis check seems stalled: https://travis-ci.org/w3c/web-platform-tests/builds/329478528?utm_source=github_status&utm_medium=notification. Perhaps you could kick it again, @Hexcles? :)

[Hexcles]

@mikewest Travis had an outage yesterday. It's now back online and the PR has been merged! :)