Use WebInspector.ChunkedFileReader to speed up heap snapshot saving

BUG=324769
R=loislo@chromium.org

Review URL: https://codereview.chromium.org/113613003

git-svn-id: svn://svn.chromium.org/blink/trunk@164318 bbb929c8-8fbe-4397-9dbb-9b2b20218538