[wasm] Disable async compilation in single-threaded mode.

Async compilation relies on background tasks, which are not available
in single-threaded mode.

Change-Id: Id47e7f5bd1406a5323d6a770936a23b6839e7936
Reviewed-on: https://chromium-review.googlesource.com/880583Reviewed-by: Andreas Haas <ahaas@chromium.org>
Commit-Queue: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#50789}

src/flag-definitions.h

@@ -1274,6 +1274,7 @@ DEFINE_BOOL(predictable, false, "enable predictable mode")
 DEFINE_IMPLICATION(predictable, single_threaded)
 DEFINE_NEG_IMPLICATION(predictable, memory_reducer)
 DEFINE_VALUE_IMPLICATION(single_threaded, wasm_num_compilation_tasks, 0)
+DEFINE_NEG_IMPLICATION(single_threaded, wasm_async_compilation)
 
 //
 // Threading related flags.