Adds a bar below the current timeline view which can show the time
when an individual function was on the stack. Functions in the call
stack are now clickable to show them in this view.

Sections where the function was on the stack, but not at the top, are
displayed at half height.

Review-Url: https://codereview.chromium.org/2737083003
Cr-Commit-Position: refs/heads/master@{#43673}