Script streaming: use an enum for the "why not streaming" UMA values.
Script streaming: use an enum for the "why not streaming" UMA values.

Makes sense to keep the UMA tracking in place; this helps us monitor streaming
over time.

R=haraken@chromium.org
BUG=

Review URL: https://codereview.chromium.org/1036823002

git-svn-id: bf5cd6ccde378db821296732a091cfbcf5285fbd@192523 bbb929c8-8fbe-4397-9dbb-9b2b20218538

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