Add a histogram for beforeunload events in subframes.

Our support for them is buggy. Also Gecko and pre-Chrome Opera
never fire beforeunload from subframes. If the usage is low
enough, maybe we can remove support for it as well.

BUG=244804

Review URL: https://chromiumcodereview.appspot.com/22322002

git-svn-id: svn://svn.chromium.org/blink/trunk@155565 bbb929c8-8fbe-4397-9dbb-9b2b20218538