With the existing platform API it is not possible to post foreground
tasks from background tasks. This is, however, required to implement
asynchronous compilation for WebAssembly. With this CL we add the
concept of a TaskRunner to the platform API. The TaskRunner contains
all data needed to post a foreground task and can be used both from a
foreground task and a background task. Eventually the TaskRunner should
replace the existing API.

In addition, this CL contains a default implementation of the
TaskRunner. This implementation has tempory workaround for platforms
which do not provide a TaskRunner implementation yet. This default
implementation should be deleted again when all platforms provide a
TaskRunner implementation.

R=rmcilroy@chromium.org

Cq-Include-Trybots: master.tryserver.chromium.linux:linux_chromium_rel_ng
Change-Id: I6ea4a1c9da1eb9a19e8ce8f2163000dbc2598802
Reviewed-on: https://chromium-review.googlesource.com/741588
Commit-Queue: Andreas Haas <ahaas@chromium.org>
Reviewed-by: Ross McIlroy <rmcilroy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#49041}