This is a small refactoring that moves the WasmCodeManager and
CompilationManager from being a part of the Isolate directly to living in a new
WasmEngine object. This makes it easier to change Wasm components without
rebuilding so much of V8, and also enables future changes to Wasm without
affecting unrelated parts of V8.

Bug: v8:7109
Cq-Include-Trybots: master.tryserver.chromium.linux:linux_chromium_rel_ng
Change-Id: Ic89bfc3974483aa909d12556d1386e18785a1d71
Reviewed-on: https://chromium-review.googlesource.com/804824
Commit-Queue: Eric Holk <eholk@chromium.org>
Reviewed-by: Ben Titzer <titzer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#49848}