[wasm][threads] Do not overwrite the V8 flag with the origin trial flag

R=titzer@chromium.org

Bug: chromium:868844
Change-Id: Ib96416dc6ae36e024e90187944f2e9ca92e8b83b
Reviewed-on: https://chromium-review.googlesource.com/1183200Reviewed-by: Ben Titzer <titzer@chromium.org>
Commit-Queue: Andreas Haas <ahaas@chromium.org>
Cr-Commit-Position: refs/heads/master@{#55262}

src/wasm/wasm-features.cc

@@ -26,7 +26,7 @@ WasmFeatures WasmFeaturesFromFlags() {
 
 WasmFeatures WasmFeaturesFromIsolate(Isolate* isolate) {
   WasmFeatures features = WasmFeaturesFromFlags();
-  features.threads =
+  features.threads |=
       isolate->AreWasmThreadsEnabled(handle(isolate->context(), isolate));
   return features;
 }