Adjust |index| into cursorable position in NativeTextfieldViews::GetCompositionCharacterBounds

RenderText::GetCursorBounds expects that that |caret| parameter is placed at somewhere cursorable so far. In fact, tt has a DCHECK like this: DCHECK(IsCursorablePosition(caret_pos)).

On the other hand, currently NativeTextfieldViews::GetCompositionCharacterBounds(index, ...) is implemented by calling RenderText::GetCursorBounds with |index| and |index + 1|.  This may cause DCHECK failure because neither |index| nor |index + 1| is guaranteed to be cursorable position.

This CL updates NativeTextfieldViews::GetCompositionCharacterBounds to make sure that RenderText::GetCursorBounds is called with cursorable position.

This CL also fixes a comment bug in RenderText::GetCursorBounds().

BUG=325083
TEST=views_unitetest --gtest_filter=NativeTextfieldViewsTest.*

Review URL: https://codereview.chromium.org/101603002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@238686 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 238686