views::Textfield: Implement yank editing command.

ui/views/controls/textfield/textfield_model.h

Index: ui/views/controls/textfield/textfield_model.h
diff --git a/ui/views/controls/textfield/textfield_model.h b/ui/views/controls/textfield/textfield_model.h
index df3e9754e2d52c1e51949b542cf3d2552c56dfc4..e63072b6d4bf7a2b0349bc9a02c9e50b60b2a751 100644
--- a/ui/views/controls/textfield/textfield_model.h
+++ b/ui/views/controls/textfield/textfield_model.h
@@ -95,15 +95,17 @@ class VIEWS_EXPORT TextfieldModel {
 
   // Deletes the first character after the current cursor position (as if, the
   // the user has pressed delete key in the textfield). Returns true if
-  // the deletion is successful.
+  // the deletion is successful. If |add_to_kill_buffer| is true, the deleted
+  // text is copied to the kill buffer.
   // If there is composition text, it'll be deleted instead.
-  bool Delete();
+  bool Delete(bool add_to_kill_buffer = false);

[msw, 2016/07/22 17:44:03]

I thought chromium generally discouraged default parameters, but I don't see
anything in our style guide, and the Google style guide says they're allowed
here (albeit with a long list of cons), so I guess this is fine.

 
   // Deletes the first character before the current cursor position (as if, the
   // the user has pressed backspace key in the textfield). Returns true if
-  // the removal is successful.
+  // the removal is successful. If |add_to_kill_buffer| is true, the deleted
+  // text is copied to the kill buffer.
   // If there is composition text, it'll be deleted instead.
-  bool Backspace();
+  bool Backspace(bool add_to_kill_buffer = false);
 
   // Cursor related methods.
 
@@ -182,6 +184,10 @@ class VIEWS_EXPORT TextfieldModel {
   // changed.
   bool Transpose();
 
+  // Pastes text from the kill buffer at the current cursor position or
+  // selection.

[msw, 2016/07/22 17:44:03]

nit: What does "or selection" mean? Just that a current selection would be
replaced, similar to the standard paste behavior? If so, you can probably nix
that tidbit, otherwise please clarify.

[karandeepb, 2016/07/25 06:38:11]

Done. Yeah it follows the standard paste behavior. 

+  bool Yank();
+
   // Tells if any text is selected, even if the selection is in composition
   // text.
   bool HasSelection() const;
@@ -223,6 +229,9 @@ class VIEWS_EXPORT TextfieldModel {
   // Clears all edit history.
   void ClearEditHistory();
 
+  // Clears the kill buffer. Exposed to clear global state between tests.
+  static void ClearKillBufferForTesting();

[sky, 2016/07/22 15:22:49]

How about friending the helper and moving this to private? Having a bunch of
FooForTesting results in an ugly API.

[karandeepb, 2016/07/25 06:38:11]

Done. But friending BridgedNativeWidgetTest here seems slightly ugly. 

[sky, 2016/07/25 15:26:27]

Another option is to friend a class that is used by all test. These are commonly
called TestApi, eg TextfieldModelTestApi.

+
  private:
   friend class internal::Edit;