Check the frame for null only if the reader is null

This change moves the frame check inside the reader check to address
cases when GamepadSharedMemoryReader::Start() is called again after
Stop(), but the frame is null. Since the frame parameter is only used
when creating a new instance of GamepadSharedMemoryReader, the check was
moved inside the corresponding condition.

Bug: 864557
Change-Id: I7b32e82f5c79af2007b211cf80cff43ff33be2c9
Reviewed-on: https://chromium-review.googlesource.com/1141156Reviewed-by: Matt Reynolds <mattreynolds@chromium.org>
Reviewed-by: Reilly Grant <reillyg@chromium.org>
Commit-Queue: Oksana Zhuravlova <oksamyt@chromium.org>
Cr-Commit-Position: refs/heads/master@{#576095}

third_party/blink/renderer/modules/gamepad/gamepad_dispatcher.cc

@@ -79,11 +79,12 @@ void GamepadDispatcher::DispatchDidConnectOrDisconnectGamepad(
 }
 
 void GamepadDispatcher::StartListening(LocalFrame* frame) {
-  // TODO(crbug.com/850619): ensure a valid frame is passed
-  if (!frame)
-    return;
-  if (!reader_)
+  if (!reader_) {
+    // TODO(crbug.com/850619): ensure a valid frame is passed
+    if (!frame)
+      return;
     reader_ = std::make_unique<GamepadSharedMemoryReader>(*frame);
+  }
   reader_->Start(this);
 }