Added license header generation to tool/generate_symbols_table.dart
Added license header generation to tool/generate_symbols_table.dart

google/webcrypto.dart,google/webcrypto.dart,google/webcrypto.dart,google/webcrypto.dart,google/webcrypto.dart,google/webcrypto.dart,google/webcrypto.dart,google/webcrypto.dart,google/webcrypto.dart