[wasm] Turn on async compilation

We extended the platform API so that it is now possible to post
foreground tasks from background tasks. This new platform API has been
implemented now in all platforms within V8, and in the gin platform in
chrome. The implementation in node.js is still missing. I don't think
there are any node.js tests which use async compilation for
WebAssembly. Therefore we could turn on async compilation again to get
canary coverage.

Bug:chromium:764313

R=titzer@chromium.org, bradnelson@chromium.org
CC=franzih@chromium.org

Change-Id: Ib7f17d84983ad73b936e1de691c18c2f7e06ca5e
Reviewed-on: https://chromium-review.googlesource.com/775338
Commit-Queue: Andreas Haas <ahaas@chromium.org>
Commit-Queue: Brad Nelson <bradnelson@chromium.org>
Reviewed-by: Brad Nelson <bradnelson@chromium.org>
Cr-Commit-Position: refs/heads/master@{#49425}

src/flag-definitions.h

@@ -480,7 +480,7 @@ DEFINE_BOOL(wasm_disable_structured_cloning, false,
             "disable wasm structured cloning")
 DEFINE_INT(wasm_num_compilation_tasks, 10,
            "number of parallel compilation tasks for wasm")
-DEFINE_BOOL(wasm_async_compilation, false,
+DEFINE_BOOL(wasm_async_compilation, true,
             "enable actual asynchronous compilation for WebAssembly.compile")
 DEFINE_BOOL(wasm_stream_compilation, false,
             "enable streaming compilation for WebAssembly")