This is a reland of 1e5b6d99

Original change's description:
> [RCS] Create thread local runtime call stats tables for worker threads
>
> Creating a runtime call stats table for each worker thread task is expensive.
> Instead we create a single table per thread, and use TLS to get the correct one
> when starting a worker thread task.
>
> In order to correctly initialize the parser, scanner and parse-info's runtime
> call stats fields, we move creation of the scanner and parser onto the
> background tasks for BackgroundCompileTask and UnoptimizedCompilationJob.
>
> Change-Id: I36064c7fb43290968620b1985cc02637b16f4232
> Reviewed-on: https://chromium-review.googlesource.com/1187522
> Reviewed-by: Toon Verwaest <verwaest@chromium.org>
> Commit-Queue: Ross McIlroy <rmcilroy@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#55448}

TBR=verwaest@chromium.org

Change-Id: I7c2f48280651d7632d99ee22d86260c72bc16185
Reviewed-on: https://chromium-review.googlesource.com/1193463
Commit-Queue: Ross McIlroy <rmcilroy@chromium.org>
Reviewed-by: Ross McIlroy <rmcilroy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#55456}