Add extern for goog. (#3417)

As of closure-compiler-v20191027.jar the compile now breaks due to a lack of ‘goog’ being defined.

../../temp_core/constants.js:24: ERROR - [JSC_UNDEFINED_VARIABLE] variable goog is undeclared
goog.provide('Blockly.constants');
^^^^

1 error(s), 0 warning(s)
Compiler exit code: 1
Compilation FAIL.

Upgrade closure compiler to v20191027

tests/compile/compile.sh

@@ -78,6 +78,7 @@ COMPILATION_COMMAND="java -jar $COMPILER --js='$BLOCKLY_ROOT/tests/compile/main.
   --js='$BLOCKLY_ROOT/blocks/**.js' \
   --js='$BLOCKLY_ROOT/generators/**.js' \
   --generate_exports \
+  --externs $BLOCKLY_ROOT/externs/goog-externs.js \
   --externs $BLOCKLY_ROOT/externs/svg-externs.js \
   --compilation_level ADVANCED_OPTIMIZATIONS \
   --language_in ECMASCRIPT5_STRICT \