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This CL migrates UnsizedMedia from feature policy to document policy.

Following web tests are being migrated:
- unsized-media.tentative.https.sub.html
- unsized-media-reporting.html

Following web tests are removed, because document policy currently
does not have a js API to list all features available.
- feature-policy-unsized-media-disabled.html
- feature-policy-unsized-media-enabled.html

Bug: 993790
Change-Id: Id5e0189c0277c940177460380970eab0015109d0
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2097028
Commit-Queue: Charlie Hu <chenleihu@google.com>
Reviewed-by: Robert Sesek <rsesek@chromium.org>
Reviewed-by: Philip Jägenstedt <foolip@chromium.org>
Reviewed-by: Ian Clelland <iclelland@chromium.org>
Cr-Commit-Position: refs/heads/master@{#750246}