Add break statement for JSDIALOGTYPE_PROMPT.

[dwarfcrank]

Building with g++ 7.2.0 fails due to fallthrough in case JSDIALOGTYPE_PROMPT in browser_widget.cc:

browser_widget.cc: In lambda function:
browser_widget.cc:476:9: error: this statement may fall through [-Werror=implicit-fallthrough=]
         }
         ^
browser_widget.cc:478:7: note: here
       default:
       ^~~~~~~

This patch adds the missing break statement to build without errors.

[cretz]

Thanks!