[wasm] Turn off async compilation

Async compilation causes problem when the isolate shuts down
concurrently.

R=hablich@chromium.org

Bug: chromium:764313
Change-Id: I35b5ba9bdb70291b53e87a5323f5408930f01ddd
Reviewed-on: https://chromium-review.googlesource.com/687076Reviewed-by: Clemens Hammacher <clemensh@chromium.org>
Commit-Queue: Andreas Haas <ahaas@chromium.org>
Cr-Commit-Position: refs/heads/master@{#48188}

src/flag-definitions.h

@@ -481,7 +481,7 @@ 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")
 DEFINE_BOOL(wasm_stream_compilation, false,
             "enable streaming compilation for WebAssembly")