The CompilationManager was introduced to manage the memory of
AsyncCompileJobs. However, by now this can be done better by the new
WasmEngine.

This CL just moves the code to wasm-engine.[h,cc] and adjusts the
callsites.

R=titzer@chromium.org

Cq-Include-Trybots: luci.chromium.try:linux_chromium_rel_ng
Change-Id: Icd2c1f19feeaa854c74e020b41e314b8ad00cea5
Reviewed-on: https://chromium-review.googlesource.com/1052109Reviewed-by: Ben Titzer <titzer@chromium.org>
Commit-Queue: Andreas Haas <ahaas@chromium.org>
Cr-Commit-Position: refs/heads/master@{#53112}