Bug 1539569 [wpt PR 16074] - Update interfaces/webgl1.idl, a=testonly

Automatic update from web-platform-tests
Update interfaces/webgl1.idl (#16074)

Source: https://github.com/tidoust/reffy-reports/blob/e47712b/whatwg/idl/webgl1.idl
Build: https://travis-ci.org/tidoust/reffy-reports/builds/511332983
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wpt-commits: c09a72017fc21d9e6a8fc48a25e2331463716062
wpt-pr: 16074

testing/web-platform/tests/interfaces/webgl1.idl

@@ -32,6 +32,7 @@ dictionary WebGLContextAttributes {
     boolean preserveDrawingBuffer = false;
     WebGLPowerPreference powerPreference = "default";
     boolean failIfMajorPerformanceCaveat = false;
+    boolean desynchronized = false;
 };
 
 [Exposed=(Window,Worker)]