DevTools: Encapsulate should-skip-breakpoint check in debugger agent.

This way we collect debugger stacks (now top call frame) in V8 event handler
only when we need it, e.g. when framework/blackboxing feature is turned on.

R=pfeldman, yurys

Review URL: https://codereview.chromium.org/285923002

git-svn-id: svn://svn.chromium.org/blink/trunk@174203 bbb929c8-8fbe-4397-9dbb-9b2b20218538