The only difference between GetNextUncompiledFunctionId +
CompileAndSchedule and FetchAndExecuteCompilationUnit is that
FetchAndExecuteCompilationUnit potentially calls a callback if
it detects that no finishing task is executing. With this CL
I replace the two functions again with
FetchAndExecuteCompilationUnit. I add a flag so that no callback
is called when the flag is not set. If no callback is called,
FetchAndExecuteCompilationUnit behaves exactly the same
as the other two functions together.

R=clemensh@chromium.org

Change-Id: I17318381eec2d17b13d0902984f2620b909c7ea0
Reviewed-on: https://chromium-review.googlesource.com/544954Reviewed-by: Clemens Hammacher <clemensh@chromium.org>
Commit-Queue: Andreas Haas <ahaas@chromium.org>
Cr-Commit-Position: refs/heads/master@{#46210}