Make EventSender::Reset() not to check drag data is null

This patch removes |DCHECK(current_drag_data_.IsNull())| to avoid crash when
test scripts don't finish mouse drag, e.g. not to call |eventSender.mouseUp()|
because of assertion failure in w3c test scripts for ease of writing
test scripts.

This patch is suggested in another CL[1].

[1] http://crrev.com/c/2148920 Make editing/selection/drag-in-iframe.html not to
crash

Bug: 707656
Change-Id: I989c1fe8cceb39566e3f8ddda364b4dc9cc060aa
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2150346
Commit-Queue: Yoshifumi Inoue <yosin@chromium.org>
Commit-Queue: Kent Tamura <tkent@chromium.org>
Reviewed-by: Kent Tamura <tkent@chromium.org>
Auto-Submit: Yoshifumi Inoue <yosin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#759193}

content/shell/test_runner/event_sender.cc

@@ -1314,7 +1314,6 @@ EventSender::EventSender(WebWidgetTestProxy* web_widget_test_proxy)
 EventSender::~EventSender() {}
 
 void EventSender::Reset() {
-  DCHECK(current_drag_data_.IsNull());
   current_drag_data_.Reset();
   current_drag_effect_ = blink::kWebDragOperationNone;
   current_drag_effects_allowed_ = blink::kWebDragOperationNone;