Moves BackgroundParsingTask to compiler.cc and renames as BackgroundCompileTask.
This moves code out api.cc and parsing/ into compiler.cc where it belongs.

BUG=v8:7311,v8:5203

Cq-Include-Trybots: master.tryserver.chromium.linux:linux_chromium_rel_ng
Change-Id: I08a72ab8a6bdb480f519d42b36875d347b801ddc
Reviewed-on: https://chromium-review.googlesource.com/919481
Commit-Queue: Ross McIlroy <rmcilroy@chromium.org>
Reviewed-by: Marja Hölttä <marja@chromium.org>
Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Reviewed-by: Mythri Alle <mythria@chromium.org>
Cr-Commit-Position: refs/heads/master@{#51421}