With a temporary intermediate step to allow adapting embedders before
getting rid of the ExpectedRuntime method altogether.

The method is being renamed to CallOnWorkerThread() as an effort to
go away from "background" nomenclature for worker threads ("background"
usually refers to a priority but worker threads are commonly used for
high priority tasks in v8).
Other CLs will follow to rename other "background" APIs.

Bug: v8:7310
Cq-Include-Trybots: luci.chromium.try:linux_chromium_rel_ng;master.tryserver.chromium.linux:linux_chromium_rel_ng
Change-Id: I2fd4eac7458708d4eacb0f4871c982a567a3865e
Reviewed-on: https://chromium-review.googlesource.com/941442
Commit-Queue: Gabriel Charette <gab@chromium.org>
Reviewed-by: Andreas Haas <ahaas@chromium.org>
Cr-Commit-Position: refs/heads/master@{#51645}