Add a debug helper showLiveDocumentInstances()

This CL adds a debug helper function |showLiveDocumentInstances()|.
You can invoke this function from gdb, e.g)
> p showLiveDocumentInstances()
to display live WebCore::Document instances.

This is useful when debugging Document leaks.

WebLeakDetector is also modified to dump live document instances when reporting leaks.

BUG=None

Review URL: https://codereview.chromium.org/374833002

git-svn-id: svn://svn.chromium.org/blink/trunk@177646 bbb929c8-8fbe-4397-9dbb-9b2b20218538