content: Move render_widget_host_view_gtk to content/ try 2.

This was r97750, which was reverted at r97756. This patch, in addition to all
the other things it did, marks OwnedWidgetGtk for export and moves it to the
ui:: namespace.

Original list of things this patch does:
- OwnedWidgetGtk now goes in ui/base/gtk/
- TruncateString moved from l10n_util:: to ui::
- GtkIMContextWrapper has part of its code split into chrome/ (IDC using code goes in RenderViewContextMenu) and the rest go in content/ (gtk using code goes with GtkIMContextWrapper).
- gtk_key_bindings_handler[_unittest] now goes in content, as it's a utility class to RenderWidgetHostGtk.

TEST=existing unit tests
Original Review URL:

Review URL:

git-svn-id: svn:// 0039d316-1c4b-4281-b951-d872f2087c98
77 files changed