RenderText: Allow setting display offset explicitly
Currently, display offset for a gfx::RenderText is computed
automatically to ensure the cursor is always visible. This CL adds the
ability to set display offset explicitly which, as an example, can be
useful in adding touch scrolling to textfields. Display offset set this
way is not permanent and will be recalculated next time a property
(text, font, alignment, etc.) is changed.
BUG=388862
Review URL: https://codereview.chromium.org/
378723003
git-svn-id: svn://svn.chromium.org/chrome/trunk/src@282308 0039d316-1c4b-4281-b951-d872f2087c98