Splits background compilation data out of ScriptStreamingData and into
BackgroundCompileTask. Also makes BackgroundCompileTask no longer a sub-class
of ScriptStreamingTask, and instead have ScriptStreamingTask delegate to a
BackgroundCompileTask.

As part of this change, we now create the CharacterStream on the main thread,
and therefore have to set the (thread-local) runtime_call_stats on the already
created CharacterStream when the BackgroundCompileTask is run on the background
thread. As such, changes to CharacterStream were needed to feed the
runtime_call_stats through appropriately.

Deprecates Source::GetCachedData and StreamedSource::GetCachedData since they are
no longer used, and the streamed source never has cached data (streaming is
suppressed if cached data is available). Also removes Utf8ChunkedStream which
is dead code.

BUG=v8:8041, v8:8015
TBR=yangguo@chromium.org

Cq-Include-Trybots: luci.chromium.try:linux_chromium_rel_ng
Change-Id: Ifcc723ebf930a1dc01135fcb70929d6168471cb3
Reviewed-on: https://chromium-review.googlesource.com/1236353
Commit-Queue: Ross McIlroy <rmcilroy@chromium.org>
Reviewed-by: Toon Verwaest <verwaest@chromium.org>
Cr-Commit-Position: refs/heads/master@{#56182}