This is a reland of 31228a69

TBR=yangguo@chromium.org,verwaest@chromium.org

Original change's description:
> [Compile] Refactor BackgroundCompileTask to enable its use by CompilerDispatcher
>
> 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}

Bug: v8:8041, v8:8015
Change-Id: Ied5132c537d4c25c6e355f289c2a9cc1f8ff98e9
Cq-Include-Trybots: luci.chromium.try:linux_chromium_rel_ng
Reviewed-on: https://chromium-review.googlesource.com/1242097
Commit-Queue: Ross McIlroy <rmcilroy@chromium.org>
Reviewed-by: Maya Lekova <mslekova@chromium.org>
Reviewed-by: Ross McIlroy <rmcilroy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#56201}