WireAndFrontendFuzzer "wait" for all commands before freeing

Otherwise the Device might fire callbacks when it is destroyed,
causing use-after-frees on the wire server.

BUG=chromium:914620
BUG=chromium:914615

Change-Id: Ic47b219b4ebe2eb5abd525741c0ff1180282c87d
Reviewed-on: https://dawn-review.googlesource.com/c/3322
Reviewed-by: Stephen White <senorblanco@chromium.org>
Reviewed-by: Kai Ninomiya <kainino@chromium.org>
Commit-Queue: Corentin Wallez <cwallez@chromium.org>

src/fuzzers/DawnWireServerAndFrontendFuzzer.cpp

@@ -53,5 +53,12 @@ extern "C" int LLVMFuzzerTestOneInput(const uint8_t* data, size_t size) {
 
     wireServer->HandleCommands(reinterpret_cast<const char*>(data), size);
 
+    // Fake waiting for all previous commands before destroying the server.
+    nullDevice.Tick();
+
+    // Destroy the server before the device because it needs to free all objects.
+    wireServer = nullptr;
+    nullDevice = nullptr;
+
     return 0;
 }