This unifies the logic used for foreground compilation and background
compilation. The main thread used a separate
{FetchAndExecuteCompilationUnit} function so far, which implements the
same as the {BackgroundCompileTask}, but without compiling in batches.

This CL removes that method, and uses the same
{ExecuteCompilationUnits} method from both foreground and background
tasks.

R=mstarzinger@chromium.org

Bug: v8:8916
Change-Id: Id30edb766c46d860a5b0ef45386af7e9fd6a1cc3
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1564191Reviewed-by: Jakob Kummerow <jkummerow@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#60790}