This CL is the first step in introducing a compilation manager for
asynchronous compile jobs in WebAssembly.

The compilation manager holds a list of currently active
AsyncCompileJobs. With the compilation manager these compile jobs get
deallocated when the isolate shuts down. Note that this CL is not enough
to provide a graceful isolate shutdown. For this we have to wait for all
compilation tasks to finish before we shut down, and we have to make the
tasks stateless. I plan to do these changes in separate CLs.

R=clemensh@chromium.org, mtrofin@chromium.org

BUG=v8:6436

Change-Id: I9a6e165dd2ef6d33944ca303fed49f7940eea7a2
Reviewed-on: https://chromium-review.googlesource.com/528079Reviewed-by: Clemens Hammacher <clemensh@chromium.org>
Commit-Queue: Andreas Haas <ahaas@chromium.org>
Cr-Commit-Position: refs/heads/master@{#45858}