Referrer Policy: Add a flag to reduce `referer` granularity by default. [1/2]

If a site hasn't set an explicit referrer policy, this runtime-enabled
flag will strip the outgoing header down to an origin. Perhaps this is
a direction we could offer as a choice to users in the future? (Note that this
implementation does not yet effect reditects. That's something to fix later
if/when we want to look more closely at this kind of approach. Nothing worth
doing for a strawman...).

Adding this flag to help evaluate the impact such a change might have.

[1/2] Blink: This patch.
[2/2] Chromium: https://codereview.chromium.org/711033002

BUG=431711

Review URL: https://codereview.chromium.org/684683003

git-svn-id: svn://svn.chromium.org/blink/trunk@185057 bbb929c8-8fbe-4397-9dbb-9b2b20218538