Add Mojo API for Blink hit testing

third_party/WebKit/public/web/WebWidget.h

Index: third_party/WebKit/public/web/WebWidget.h
diff --git a/third_party/WebKit/public/web/WebWidget.h b/third_party/WebKit/public/web/WebWidget.h
index 101367b9cc0afa0fae2a1d2189b1a69153c0492d..147f034417acc50975783e42c8296f08205af723 100644
--- a/third_party/WebKit/public/web/WebWidget.h
+++ b/third_party/WebKit/public/web/WebWidget.h
@@ -34,6 +34,7 @@
 #include "public/platform/WebBrowserControlsState.h"
 #include "public/platform/WebCanvas.h"
 #include "public/platform/WebCommon.h"
+#include "public/platform/WebFloatPoint.h"
 #include "public/platform/WebFloatSize.h"
 #include "public/platform/WebInputEventResult.h"
 #include "public/platform/WebMenuSourceType.h"
@@ -42,6 +43,7 @@
 #include "public/platform/WebSize.h"
 #include "public/platform/WebTextInputInfo.h"
 #include "public/web/WebCompositionUnderline.h"
+#include "public/web/WebFrame.h"
 #include "public/web/WebRange.h"
 #include "public/web/WebTextDirection.h"
 
@@ -119,6 +121,11 @@ class WebWidget {
   // on receiving this message
   virtual void ThemeChanged() {}
 
+  // Finds out the frame that the point is positioned on.
+  virtual WebFrame* HitTestFrameAt(const WebFloatPoint& point) {

[bokan, 2017/07/18 19:26:35]

Would we generalise this a little? Instead of hit testing for Frames, Scroll
Unification
https://docs.google.com/document/d/1op5USoxDnN6yxB8EiFHYcGHacrZZRVKVqu4mSXFd6...)
will want the actual ElementId. Could we instead call this HitTestAt and return
a WebHitTestResult, possibly adding a method on there to return the WebFrame we
hit? I could then reuse this for the unification work.

[Navid Zolghadr, 2017/07/18 19:34:14]

I had something like that in mind. But I saw no method on WebHitTestResult to
return the frame thing and I didn't think it was worth digging more into. Now
that it seems it has a legit use case I'll take a closer look to see whether I
can manage with returning a WebHitTestResult.

[bokan, 2017/07/18 19:39:43]

Great, thanks! From the WebNode you can get a WebDocument which can give you the
WebLocalFrame. Alternatively you can just add a method right on
WebHitTestResult.

+    return nullptr;
+  }
+
   // Called to inform the WebWidget of an input event.
   virtual WebInputEventResult HandleInputEvent(const WebCoalescedInputEvent&) {
     return WebInputEventResult::kNotHandled;