[TaskScheduler]: TaskSchedulerImpl holds a single foreground_pool pointer.

Previously, TaskSchedulerImpl had 2 members foreground_pool_ and
native_foreground_pool_. This CL transforms these into a single pointer to
SchedulerWorkerPool, thus hiding implementation in source file.

Change-Id: I521e34e5a6ae72fa790b65be966b6919b9fff0f0
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1554288
Commit-Queue: Etienne Pierre-Doray <etiennep@chromium.org>
Reviewed-by: François Doray <fdoray@chromium.org>
Cr-Commit-Position: refs/heads/master@{#653596}
4 files changed