DevTools: Add MutationObserver instrumentation for async stacks tracking.

Bind async call stack for a MutationCallback call to the first point in the
code that enqueued a MutationRecord into a given MutationObserver instance.

For example, while paused in a MutationObserver callback, we will show an
async call stack pointing to the earliest place in the code where one of the
received DOM mutations happened (usually the first one).

BUG=272416
R=yurys

Review URL: https://codereview.chromium.org/119613004

git-svn-id: svn://svn.chromium.org/blink/trunk@164358 bbb929c8-8fbe-4397-9dbb-9b2b20218538