[wasm] Turn on async compilation again

With https://chromium-review.googlesource.com/c/532993/ the isolate
can shutdown without a crash even when an async compile job is still
active. This should allow us to turn on async compilation again.

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

Change-Id: I822112c4f2a7a32133aa1d086881ed77baf42f76
Reviewed-on: https://chromium-review.googlesource.com/533293Reviewed-by: Ben Titzer <titzer@chromium.org>
Commit-Queue: Andreas Haas <ahaas@chromium.org>
Cr-Commit-Position: refs/heads/master@{#46222}

src/flag-definitions.h

@@ -524,7 +524,7 @@ 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")
 // Parallel compilation confuses turbo_stats, force single threaded.
 DEFINE_VALUE_IMPLICATION(turbo_stats, wasm_num_compilation_tasks, 0)