[PE] Avoid integer overflow in WebPluginContainerImpl::Paint()

Bug: 822190
Change-Id: I63fd40f80f243dbf4a40baac940f2dd999d3e443
Reviewed-on: https://chromium-review.googlesource.com/964541Reviewed-by: Philip Rogers <pdr@chromium.org>
Commit-Queue: Xianzhu Wang <wangxianzhu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#544184}

third_party/WebKit/Source/core/exported/WebPluginContainerImpl.cpp

@@ -188,10 +188,9 @@ void WebPluginContainerImpl::Paint(GraphicsContext& context,
 
   // The plugin is positioned in the root frame's coordinates, so it needs to
   // be painted in them too.
-  IntPoint origin = ParentFrameView().ContentsToRootFrame(IntPoint(0, 0));
-  origin -= paint_offset;
-  context.Translate(static_cast<float>(-origin.X()),
-                    static_cast<float>(-origin.Y()));
+  FloatPoint origin(ParentFrameView().ContentsToRootFrame(IntPoint()));
+  origin.Move(-paint_offset);
+  context.Translate(-origin.X(), -origin.Y());
 
   WebCanvas* canvas = context.Canvas();