DevTools: Implement a workaround for missing stacks in CPU profiler

Sometimes sampler is not able to parse the JS stack and returns
a (program) sample instead. The issue leads to call frames belong
to the same function invocation being split apart.
Here's a workaround for that. When there's a single (program) sample
between two call stacks sharing the same bottom node, it is replaced
with the preceeding sample.

Review URL: https://codereview.chromium.org/245153002

git-svn-id: svn://svn.chromium.org/blink/trunk@172388 bbb929c8-8fbe-4397-9dbb-9b2b20218538