[Device Service] Move Gamepad Blink headers to be part of the Gamepad client library

This CL follows the approach of
https://codereview.chromium.org/2415083002
and moves
//third_party/WebKit/public/platform/WebGamepad.h
//third_party/WebKit/public/platform/WebGamepads.h
to
//device/gamepad/public/cpp/
and puts them in a special target to indicate that they're shared with
Blink

BUG=695012

Review-Url: https://codereview.chromium.org/2808093006
Cr-Commit-Position: refs/heads/master@{#465129}
diff --git a/content/renderer/render_thread_impl.h b/content/renderer/render_thread_impl.h
index 35c36aa..4ee6d4f7f 100644
--- a/content/renderer/render_thread_impl.h
+++ b/content/renderer/render_thread_impl.h
@@ -62,7 +62,6 @@
 namespace scheduler {
 class WebThreadBase;
 }
-class WebGamepads;
 class WebMediaStreamCenter;
 class WebMediaStreamCenterClient;
 }
@@ -79,6 +78,10 @@
 class TaskGraphRunner;
 }
 
+namespace device {
+class Gamepads;
+}
+
 namespace discardable_memory {
 class ClientDiscardableSharedMemoryManager;
 }
@@ -461,7 +464,7 @@
   }
 
   // Retrieve current gamepad data.
-  void SampleGamepads(blink::WebGamepads* data);
+  void SampleGamepads(device::Gamepads* data);
 
   // Called by a RenderWidget when it is created or destroyed. This
   // allows the process to know when there are no visible widgets.