Add "Drag And Drop Across Frames" keyword for cross-frame drag and drop support #49

Job	Run time
test_config (previous, 3.12.12, 7.3.2, 4.38.0, chrome)	5m 47s
test_config (latest, 3.13.10, 7.4.1, 4.39.0, chrome)	5m 54s
test_config (older_rf_version, 3.11.14, 6.1.1, 4.37.0, chrome)	5m 47s
	17m 28s

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