Script streaming: add UMA for tracking the reason why we don't stream.
Script streaming: add UMA for tracking the reason why we don't stream.

This will make it possible to see whether it's possible to stream more scripts
than currently (and what needs to be done to do that).

Suspecting that the biggest reason for not streaming is that the resource is
already loaded (which is OK, no actions needed there).

BUG=

Review URL: https://codereview.chromium.org/1031623002

git-svn-id: bf5cd6ccde378db821296732a091cfbcf5285fbd@192362 bbb929c8-8fbe-4397-9dbb-9b2b20218538

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