Fix mouse capture inside PDFs.

In crrev.com/6e8f1f65 a SetMouseCapture call was missed. Causing the
old issue to regress. Ensure the SetMouseCapture call is there so the
targeting of mouse events is correct during drag.

BUG=1149432

Change-Id: Ie61c25e45ac83d295111088bd8c1e25080c84a08
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2541202
Commit-Queue: Dave Tapuska <dtapuska@chromium.org>
Reviewed-by: David Bokan <bokan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#827931}

third_party/blink/renderer/core/frame/web_frame_widget_base.cc

@@ -621,6 +621,7 @@ void WebFrameWidgetBase::HandleMouseDown(LocalFrame& main_frame,
         hit_node->GetLayoutObject()->IsEmbeddedObject() && html_element &&
         html_element->IsPluginElement()) {
       mouse_capture_element_ = To<HTMLPlugInElement>(hit_node);
+      SetMouseCapture(true);
       TRACE_EVENT_NESTABLE_ASYNC_BEGIN0("input", "capturing mouse",
                                         TRACE_ID_LOCAL(this));
     }
@@ -1740,8 +1741,10 @@ WebInputEventResult WebFrameWidgetBase::HandleCapturedMouseEvent(
   HTMLPlugInElement* element = mouse_capture_element_;
 
   // Not all platforms call mouseCaptureLost() directly.
-  if (input_event.GetType() == WebInputEvent::Type::kMouseUp)
+  if (input_event.GetType() == WebInputEvent::Type::kMouseUp) {
+    SetMouseCapture(false);
     MouseCaptureLost();
+  }
 
   AtomicString event_type;
   switch (input_event.GetType()) {