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Getting scopes info from V8 debugger takes significant time.
Sometimes we don't need the scopes at all, thus we can save on this expensive work.
For async stacks we will be getting "fast" scope chains: up to the Local scope only (coming soon).

R=yurys

Review URL: https://codereview.chromium.org/208263015

git-svn-id: svn://svn.chromium.org/blink/trunk@169854 bbb929c8-8fbe-4397-9dbb-9b2b20218538