Add an histogram to track the number of beforeunload/unload events

https://chromiumcodereview.appspot.com/69083004/ added an histogram to record
the frequency of beforeunload/unload events in Blink. This is the corresponding
change in histogram.xml.

BUG=142458

Review URL: https://codereview.chromium.org/85343003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@237331 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 237331