Recognize WEBGL extensions on Firefox

tests/html/html.status

Index: tests/html/html.status
diff --git a/tests/html/html.status b/tests/html/html.status
index 0f917c5498ecb9040868abac614c4f4bc653f651..a9f94d18f434f822d69e57b75267c4203634635a 100644
--- a/tests/html/html.status
+++ b/tests/html/html.status
@@ -341,8 +341,12 @@ websql_test/supported: Fail
 [ $compiler == dart2js && ($runtime == drt || $runtime == ff) ]
 request_animation_frame_test: Skip # Async test hangs.
 
+[ $runtime == drt ]
+webgl_extensions_test: Skip # webgl does not work properly on DRT, which is 'headless'.
+
 [ $compiler == dart2js && $csp && ($runtime == drt || $runtime == safari || $runtime == ff || $runtime == chrome || $runtime == chromeOnAndroid) ]
 # Note: these tests are all injecting scripts by design.  This is not allowed under CSP.
+# TODO(sra): Change these tests to use a same-origin JavaScript script file.
 event_customevent_test: SkipByDesign
 js_interop_1_test: SkipByDesign
 js_test: SkipByDesign