[ScriptStreaming] Fix incorrect DCHECK.

ready_state_ could be kErrorOccured as well as kReadyStreaming. Update the
logic to check for this and add a new NotStreamingReason to log it.

BUG=chromium:865098,chromium:871276

Change-Id: I7f4e17c843b74632d4cf167664f26fdb92f30a8c
Reviewed-on: https://chromium-review.googlesource.com/1164943
Commit-Queue: Ross McIlroy <rmcilroy@chromium.org>
Reviewed-by: Kouhei Ueno <kouhei@chromium.org>
Cr-Commit-Position: refs/heads/master@{#581882}
4 files changed