Enable checker.py to execute type-checking almost all of the JavaScript files at once

[hajimehoshi]

This CL enables checker.py to execute type-checking almost all of the JavaScript files at once. We can execute this just with checker.cmd or checker.sh without any arguments.

I eliminated testing/*.js and polyfill.js from the set of the target because they cause a lot of warnings which seem difficult to solve.

This execution takes much less time than when we execute every test separately.

PTAL

[hajimehoshi]

$ time CR_ROOT=~/chromium_webkit/src ./checker.sh content_model.js editing_context.js editor.js operations.js read_only_selection.js selection_tracker.js
real    0m13.021s
user    1m39.797s
sys     0m3.712s

$ time CR_ROOT=~/chromium_webkit/src ./checker.sh
real    0m5.063s
user    0m25.884s
sys     0m0.778s

Note: the latter also executes commands/*.js.