Add unsized-media to feature policy.

unsized-media sets a default size for laying out intrinsically sized
images / videos.
The feature is allowed for all origins by default.
When disabled, any intrinsically sized images / videos will be using
the default dimension (300 x 150) to prevent relayout.

Bug: 623682
Change-Id: I1bd105ded2580349e2a2b177e87b70417403cb59
Reviewed-on: https://chromium-review.googlesource.com/865159
Reviewed-by: Chris Harrelson <chrishtr@chromium.org>
Reviewed-by: Robert Sesek <rsesek@chromium.org>
Reviewed-by: Ian Clelland <iclelland@chromium.org>
Commit-Queue: Luna Lu <loonybear@chromium.org>
Cr-Commit-Position: refs/heads/master@{#532496}
16 files changed