Allowing stepping into custom element lifecycle callbacks from the debugger.

Added a new inspector callback which lets the debugger know that it can pause on the next event if it needs to.

BUG=329659

Review URL: https://codereview.chromium.org/119613002

git-svn-id: svn://svn.chromium.org/blink/trunk@164621 bbb929c8-8fbe-4397-9dbb-9b2b20218538