Plumb gesture source value through Blink.

content/renderer/input/input_handler_proxy.cc

Index: content/renderer/input/input_handler_proxy.cc
diff --git a/content/renderer/input/input_handler_proxy.cc b/content/renderer/input/input_handler_proxy.cc
index b5dcb181261f6f8b19f7d1a13c7dfc73caa41b4b..b110bf2e6f8ee08536960a871ae6eac34a65a796 100644
--- a/content/renderer/input/input_handler_proxy.cc
+++ b/content/renderer/input/input_handler_proxy.cc
@@ -500,7 +500,6 @@ InputHandlerProxy::EventDisposition InputHandlerProxy::HandleGestureScrollEnd(
 InputHandlerProxy::EventDisposition InputHandlerProxy::HandleGestureFlingStart(
     const WebGestureEvent& gesture_event) {
   cc::InputHandler::ScrollStatus scroll_status;
-
   if (gesture_event.sourceDevice == blink::WebGestureDeviceTouchpad) {

[tdresser, 2015/10/14 15:14:20]

This might read a bit better as a switch statement now, but I don't feel
strongly about it.

[wjmaclean, 2015/10/14 15:55:09]

Done.

     if (gesture_event.data.flingStart.targetViewport) {
       scroll_status = input_handler_->RootScrollBegin(
@@ -510,11 +509,14 @@ InputHandlerProxy::EventDisposition InputHandlerProxy::HandleGestureFlingStart(
           gfx::Point(gesture_event.x, gesture_event.y),
           cc::InputHandler::NON_BUBBLING_GESTURE);
     }
-  } else {
+  } else if (gesture_event.sourceDevice == blink::WebGestureDeviceTouchscreen) {
     if (!gesture_scroll_on_impl_thread_)
       scroll_status = cc::InputHandler::SCROLL_ON_MAIN_THREAD;
     else
       scroll_status = input_handler_->FlingScrollBegin();
+  } else {  // sourceDevice == blink::WebGestureDeviceUninitialazed.
+    NOTREACHED();
+    return DID_NOT_HANDLE;
   }
 
 #ifndef NDEBUG
@@ -1012,6 +1014,9 @@ bool InputHandlerProxy::scrollBy(const WebFloatSize& increment,
   bool did_scroll = false;
 
   switch (fling_parameters_.sourceDevice) {
+    case blink::WebGestureDeviceUninitialized:
+      NOTREACHED();
+      break;

[tdresser, 2015/10/14 15:14:20]

We generally put exceptional cases at the end of switch statements.

[wjmaclean, 2015/10/14 15:55:09]

Done.

     case blink::WebGestureDeviceTouchpad:
       did_scroll = TouchpadFlingScroll(clipped_increment);
       break;