[wasm] Disable async compilation.

This doesn't fix the bug, just avoids it.

Bug: v8:6436
Change-Id: I06305a9baf892e4039f2aaf353fa7edf7b7e325d
Reviewed-on: https://chromium-review.googlesource.com/513242Reviewed-by: Brad Nelson <bradnelson@chromium.org>
Commit-Queue: Brad Nelson <bradnelson@chromium.org>
Cr-Commit-Position: refs/heads/master@{#45499}

src/flag-definitions.h

@@ -542,7 +542,7 @@ 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, true,
+DEFINE_BOOL(wasm_async_compilation, false,
             "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)