Add another way to detect whether a viewer is expected. (ampproject#6038

)

Further explain the details for the autoplay attribute (ampproject#6025)

Fix error: "Can't find variable: TextDecoder" (ampproject#6011)

* Update amp-ad-network-doubleclick-impl.md

* Reverting doc changes

The previous change was made in error (wrong branch).

* Initial changes, need to update tests

* Start fixing tests

* Fix unit test coverage

* revert a4a examples

* Include self (window) when verifying TextDecoder/TextEncoder existence

* fix upstream conflict

* PR feedback

* PR feedback

Don't throw on sessionStorage non-availability. (ampproject#5968)

This is common (e.g. in private browsing mode) and completely fine.

Fixes ampproject#5964

Do not use new iOS viewport in iOS7 and lower. (ampproject#5959)

Because it doesn't allow overriding `document.body`.

Integration tests fix and forcing running tests when files in example…

…s folder change (ampproject#5851)

Fixes a4a should validate test failure (ampproject#5847)

Fixes a4a should validate test failure (ampproject#5847)

Fix typo and spacing (ampproject#5739)

Allow webviews to be set as trusted viewers (ampproject#5592)

* Allow webviews to be set as trusted viewers

Webviews can't set `ancestorOrigins` properly (maybe?), we can't use it
to tell if we are in a trusted viewer context. Instead, fall back to our
"old browser" path, which creates a `trustedViewerResolver_`. When the
webview's integration script [sets the message
deliverer](https://github.com/ampproject/amphtml/blob/f28e116/src/service/viewer-impl.js#L947),
it [will
resolve](https://github.com/ampproject/amphtml/blob/f28e116/src/service/viewer-impl.js#L961-L962)
to the webview's passed origin.

Fixes ampproject#5563.

* Add tests

* Do not trust "webviews" that are really bad actor iframes

We treat them like normal iframes.

* Test for '1' explicitly

* Fix test