Add Mojo API for Blink hit testing

content/common/input/input_handler.mojom

Index: content/common/input/input_handler.mojom
diff --git a/content/common/input/input_handler.mojom b/content/common/input/input_handler.mojom
index fae10f96c1d3d7eacec511775e4f7800e2e404a8..31b8aa5e70fe8fe94e9fbfa9f2e2f65b6f48890b 100644
--- a/content/common/input/input_handler.mojom
+++ b/content/common/input/input_handler.mojom
@@ -58,6 +58,9 @@ interface FrameInputHandler {
   SelectAll();
   CollapseSelection();
 
+  // Returns the hit-tested remote frame id for the given coordinates.

[rjkroege, 2017/06/26 18:29:52]

how are we suppose to use this API from ui/ws? Couldn't this be a component?

[Navid Zolghadr, 2017/07/17 18:54:45]

riajiang@ I believe you have written some code that expect to use this code
based on what Robert said before. Can you point me to that code so I can work on
exposing this API to that part of the code?

+  HitTestFrameAt(gfx.mojom.PointF point) => (int32 frame_id);

[rjkroege, 2017/06/26 18:29:52]

What is your expectation of the coordinate system of point? I've been presuming
that the CTS of point is the CTS of the target frame?

API seems reasonable to me.

[riajiang, 2017/06/27 16:37:28]

Input |point| would be in display-coordinate-pixels (or I can convert it to
screen-coordinates before asking blink) and we need to update it to be in
target's CTS after we found the target |frame_id|?

[rjkroege, 2017/06/28 18:40:19]

display coordinate pixels is  likely wrong. I claim x,y w.r.t. the target frame

[Navid Zolghadr, 2017/06/28 19:25:06]

Essentially you are asking frame X to tell you the target frame of a point. That
point should be translated to the coordinate space of Frame X. Meaning that if
Frame X was rotated in its parent then it has no idea that it happened and you
need to transform the event accordingly for that frame X understands it.

+
   // Replaces the selected region or a word around the cursor with the
   // specified string.
   Replace(mojo.common.mojom.String16 word);