DevTools: More lazy initialization when getting debugger call stacks.

Speed improvement up to 20%.

BUG=340285
R=yurys

Review URL: https://codereview.chromium.org/198253003

git-svn-id: svn://svn.chromium.org/blink/trunk@169237 bbb929c8-8fbe-4397-9dbb-9b2b20218538