HeapProfiler: Show string content as node name for concatenated strings.

ConsString is not a string from internal point of view.
But users would like to see the content of these strings.
So we reconstructs the content of these strings lazily.

The binary tree of the ConsStrings could be very deep
so we can't use the recursion here.

BUG=none

Review URL: https://chromiumcodereview.appspot.com/23868019

git-svn-id: svn://svn.chromium.org/blink/trunk@157680 bbb929c8-8fbe-4397-9dbb-9b2b20218538