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.

Relanding r157080 with test fixes.

BUG=None
TBR=loislo@chromium.org

Review URL: https://codereview.chromium.org/23532031

git-svn-id: svn://svn.chromium.org/blink/trunk@157109 bbb929c8-8fbe-4397-9dbb-9b2b20218538