Why are we doing this? Will not iframes start decoding and producing the sound programmatically?

How could an iframe produce a sound programmatically? The idea is that it shouldn't be possible without a user gesture.

Actually we have already applied the same restriction to WebAudio, so a cross-origin iframe is not able to play sound programmatically without a user gesture.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/df72ed087b3083bdea9b882bf473f8a9f4dc1276

commit df72ed087b3083bdea9b882bf473f8a9f4dc1276
Author: zqzhang <zqzhang@chromium.org>
Date: Thu Dec 08 16:07:55 2016

Adding an experimental flag to block autoplay with sound in cross-origin iframes

This CL adds an experimental flag to block autoplay with sound in
cross-origin iframes on Desktop. The experimental flag is
disabled by default, and will override
the "MediaPlaybaRequiresUserGesture" flag.

Currently, the default behavior is unchanged, and a follow-up CL
will collect metrics pretending the flag is on.

Design doc:
https://docs.google.com/a/google.com/document/d/1g05pahL0py0TSWIDwli1MLJiRuuJoleitl572DGGESs/edit?usp=sharing

BUG= 665456 

Review-Url: https://codereview.chromium.org/2496593002
Cr-Commit-Position: refs/heads/master@{#437254}

[modify] https://crrev.com/df72ed087b3083bdea9b882bf473f8a9f4dc1276/chrome/app/generated_resources.grd
[modify] https://crrev.com/df72ed087b3083bdea9b882bf473f8a9f4dc1276/chrome/browser/about_flags.cc
[modify] https://crrev.com/df72ed087b3083bdea9b882bf473f8a9f4dc1276/content/browser/renderer_host/render_view_host_impl.cc
[modify] https://crrev.com/df72ed087b3083bdea9b882bf473f8a9f4dc1276/content/child/runtime_features.cc
[modify] https://crrev.com/df72ed087b3083bdea9b882bf473f8a9f4dc1276/content/public/common/common_param_traits_macros.h
[modify] https://crrev.com/df72ed087b3083bdea9b882bf473f8a9f4dc1276/content/public/common/content_features.cc
[modify] https://crrev.com/df72ed087b3083bdea9b882bf473f8a9f4dc1276/content/public/common/content_features.h
[modify] https://crrev.com/df72ed087b3083bdea9b882bf473f8a9f4dc1276/content/public/common/web_preferences.cc
[modify] https://crrev.com/df72ed087b3083bdea9b882bf473f8a9f4dc1276/content/public/common/web_preferences.h
[modify] https://crrev.com/df72ed087b3083bdea9b882bf473f8a9f4dc1276/content/renderer/render_view_impl.cc
[add] https://crrev.com/df72ed087b3083bdea9b882bf473f8a9f4dc1276/third_party/WebKit/LayoutTests/http/tests/media/autoplay-crossorigin.html
[add] https://crrev.com/df72ed087b3083bdea9b882bf473f8a9f4dc1276/third_party/WebKit/LayoutTests/http/tests/media/resources/autoplay-crossorigin-iframe.html
[modify] https://crrev.com/df72ed087b3083bdea9b882bf473f8a9f4dc1276/third_party/WebKit/Source/core/frame/Settings.in
[modify] https://crrev.com/df72ed087b3083bdea9b882bf473f8a9f4dc1276/third_party/WebKit/Source/core/html/HTMLMediaElement.cpp
[modify] https://crrev.com/df72ed087b3083bdea9b882bf473f8a9f4dc1276/third_party/WebKit/Source/web/WebSettingsImpl.cpp
[modify] https://crrev.com/df72ed087b3083bdea9b882bf473f8a9f4dc1276/third_party/WebKit/Source/web/WebSettingsImpl.h
[modify] https://crrev.com/df72ed087b3083bdea9b882bf473f8a9f4dc1276/third_party/WebKit/public/web/WebSettings.h
[modify] https://crrev.com/df72ed087b3083bdea9b882bf473f8a9f4dc1276/tools/metrics/histograms/histograms.xml