Add window.internals.observeGC for leak-related tests.

The objective of this change is to make it possible to write unit
tests to detect leaks. The approach is to add a test hook, used like
this:

  var observation = window.internals.observeGC(value);

When value is "near death" during a garbage collection (perhaps
triggered in a test by calling gc()) then

  observation.wasCollected

is set to true. (Of course, it behooves the test writer not to create
a reference from observation to value.)

The reliability of this as a testing method is unproven. gc()
reportedly does seven garbage collections, so I think it will be
useful. The plan is to use this in Custom Elements leak tests; those
tests can be a litmus test for whether to keep this hook.

BUG=none
R=abarth@chromium.org, adamk@chromium.org

Review URL: https://codereview.chromium.org/21046002

git-svn-id: svn://svn.chromium.org/blink/trunk@155188 bbb929c8-8fbe-4397-9dbb-9b2b20218538