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/1187522Reviewed-by: Toon Verwaest <verwaest@chromium.org>
Commit-Queue: Ross McIlroy <rmcilroy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#55448}