Bug 1556623 [wpt PR 17157] - Update interfaces/media-capabilities.idl, a=testonly

Automatic update from web-platform-tests
Update interfaces/media-capabilities.idl (#17157)

Source: https://github.com/tidoust/reffy-reports/blob/14cf7a6/whatwg/idl/media-capabilities.idl
Build: https://travis-ci.org/tidoust/reffy-reports/builds/541081781
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wp5At-commits: 33a405b391908783906ab829d13cd3d691708a1b
wpt-pr: 17157

testing/web-platform/tests/interfaces/media-capabilities.idl

@@ -33,6 +33,7 @@ dictionary VideoConfiguration {
   required unsigned long height;
   required unsigned long long bitrate;
   required DOMString framerate;
+  boolean hasAlphaChannel;
 };
 
 dictionary AudioConfiguration {