[blink] Use scheduler for script streaming

Add a new feature flag, ScheduledScriptStreaming, which makes the script
streamer use the task scheduler for streaming tasks, rather than using a
dedicated ScriptStreamerThread.

Bug: chromium:865098
Bug: chromium:866868
Change-Id: I05ca411d8d5187c4b5180011ebce2a603e3944fe
Reviewed-on: https://chromium-review.googlesource.com/1145386
Commit-Queue: Leszek Swirski <leszeks@chromium.org>
Reviewed-by: Kinuko Yasuda <kinuko@chromium.org>
Reviewed-by: Kouhei Ueno <kouhei@chromium.org>
Cr-Commit-Position: refs/heads/master@{#577543}
9 files changed