Report more details in heap snapshot progress indicator

When loading heap snapshot (either from file or backend) report separate progress for loading nodes, edges, strings and each processing step.

BUG=None
R=loislo@chromium.org

Review URL: https://codereview.chromium.org/23717022

git-svn-id: svn://svn.chromium.org/blink/trunk@157080 bbb929c8-8fbe-4397-9dbb-9b2b20218538