DevTools: Optimize async call stacks: make function mirror lazily.

Speed improvement up to 10%.

BUG=340285
R=yurys

Review URL: https://codereview.chromium.org/200333002

git-svn-id: svn://svn.chromium.org/blink/trunk@169338 bbb929c8-8fbe-4397-9dbb-9b2b20218538