text-combine: horizontal + text-transform doesn't work

When we transform text, we set transformed text to
RenderText::m_text. RenderCombineText overwrites the text at
combineText() and use originalText() at getStringToRender() in
painting. Therefore, text combine paints original text instead of
transformed text.

We should save the m_text and return the text in getStringToRender().

BUG=345219

Review URL: https://codereview.chromium.org/173623003

git-svn-id: svn://svn.chromium.org/blink/trunk@167624 bbb929c8-8fbe-4397-9dbb-9b2b20218538