This is based on the original CL http://codereview.chromium.org/3142008.
The key difference is
* This uses Textfield framework and NativeTextfieldView implements NativeTextfieldWrapper.
  This allows us to swap the implementation without recompling the tree and can start
  testing on bots.
* Changed the name of the model to TextfieldViewModel as TextfieldModel may be confusing
  as other Textfield implementations are not using it. I also changed to use string16 instead
  of gap buffer as it's enough for single line text. We can update the model to use GapBuffer when necessary.
* Changed to use string16 as that's what chrome codebase should use.
* Added a switch to turn on TextfieldView.

I also filled a couple of features such as:
  * selection by key
  * mouse actions (move cursor, selection)
  * used WordIterator, which is i18n compatible, to move cursor by word
  * blinking cursor

This is only for linux based build due to KeyStroke difference.

I'm going to move some of test utlity function in chrome/browser/automation/ui_controls to app/test
and will add more test once the migration is done.

BUG=none
TEST=new unit tests are added : NativeTestfieldViewTest and TextfieldViewModelTest.

Review URL: http://codereview.chromium.org/5857002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@69523 0039d316-1c4b-4281-b951-d872f2087c98