Add dedicated windowToViewport method

third_party/WebKit/public/web/WebWidgetClient.h

Index: third_party/WebKit/public/web/WebWidgetClient.h
diff --git a/third_party/WebKit/public/web/WebWidgetClient.h b/third_party/WebKit/public/web/WebWidgetClient.h
index fa5949db75f6f3d03f1d47eb7e6b1c0b787648c4..2b597eb65b9caa0c547cbc8804c5bc528ccc9b44 100644
--- a/third_party/WebKit/public/web/WebWidgetClient.h
+++ b/third_party/WebKit/public/web/WebWidgetClient.h
@@ -184,6 +184,13 @@ public:
     // TODO(oshima): Update the comment when the migration is completed.
     virtual void convertViewportToWindow(WebRect* rect) {}
 
+    // Converts the |scalar| from the coordinates in native window in
+    // DIP to Blink's Viewport coordinates. They're identical in
+    // tradional world, but will differ when use-zoom-for-dsf feature
+    // is eanbled.  TODO(oshima): Update the comment when the
+    // migration is completed.
+    virtual float convertWindowToViewport(float scalar) { return scalar; }

[dgozman, 2016/01/23 00:38:38]

I think we should either make this symmetrical to |convertViewportToWindow| and
operate on WebRect, or call it |windowToViewportScaleFactor| and let clients
multiply themselves as some of them already do.

+
 protected:
     ~WebWidgetClient() { }
 };