Add an histogram to record the number of unload event listeners

Waiting for the unload event on navigations is a performance hit on Android.
This records the number of unload event listeners executed, to assess how many
of these waits are actually worthwhile.

BUG=142458

Review URL: https://codereview.chromium.org/69083004

git-svn-id: svn://svn.chromium.org/blink/trunk@162617 bbb929c8-8fbe-4397-9dbb-9b2b20218538