[Bringup] Handle nonexistent function in chromium v56.0.2924.0

[Bringup] Handle nonexistent function in chromium v56.0.2924.0

The SystemDragEnded's parameters are different
between chromium v56.0.2924.0 and v56.0.2924.87.

Following patches need to be migarated from opensource to chromium-efl.
Until then, this guards the function.

Drag-and-drop: Target drag messages to specific RenderWidgets.
https://codereview.chromium.org/2505113002

Drag-and-drop: Target drag messages (the sequel).
https://codereview.chromium.org/2509933002

Change-Id: I1d5aa24826936d30fc293c67380dda8cd1aacb08
Signed-off-by: Youngsoo Choi <kenshin.choi@samsung.com>