[chromedriver] Change JavaScriptDialogManager to listen for Page.javascriptDialogClosed instead of Page.javascriptDialogClosing

R=samuong@chromium.org,jbudorick@chromium.org,stgao@chromium.org
BUG=chromedriver:537

Review URL: https://codereview.chromium.org/812553006

Cr-Commit-Position: refs/heads/master@{#309302}