Places a small array in PendingTask that records the past 4 PostTasks
that result in this task being run. In essence, it creates a backtrace
of the asynchronous call stack resulting in this task being run.

This array is copied onto the stack during execution so that it is
available in crash dumps.

BUG=690197

Review-Url: https://codereview.chromium.org/1044413002
Cr-Commit-Position: refs/heads/master@{#449471}