Do not override data getter in heap snapshot grid nodes

This complicates things significantly and also leads to repeated computations of the data object for each cell.

BUG=None

Review URL: https://codereview.chromium.org/211273005

git-svn-id: svn://svn.chromium.org/blink/trunk@170042 bbb929c8-8fbe-4397-9dbb-9b2b20218538