Script streaming: better histograms.

Track the whether the script was streamed more accuratery (as streaming can be
suppressed when the first data chunk arrives).

Track whether the streaming was started separately for different types of scripts.

R=haraken@chromium.org
BUG=

Review URL: https://codereview.chromium.org/596293002

git-svn-id: svn://svn.chromium.org/blink/trunk@182696 bbb929c8-8fbe-4397-9dbb-9b2b20218538