Add metrics for tracking time between "script loaded" and "script compiled".

The histograms will be used for monitoring the effects of script
streaming (parsing and loading simultaneously).

R=jochen@chromium.org
BUG=

Review URL: https://codereview.chromium.org/531693002

git-svn-id: svn://svn.chromium.org/blink/trunk@181225 bbb929c8-8fbe-4397-9dbb-9b2b20218538