This is a refactoring CL in preparation for implementing asynchronous compilation.
It moves all the compilation-related functionality to a helper class which will
make it easier to split into background and foreground tasks which is needed
for concurrent/synchronous execution of compilation.

R=clemensh@chromium.org,ahaas@chromium.org
BUG=v8:6003

Review-Url: https://codereview.chromium.org/2710603006
Cr-Commit-Position: refs/heads/master@{#43413}