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RenderText previously only had a SetText to alter the text.  Calling SetText would reset the text styling already setup.

It can be more efficient in some cases to preserve the existing font styling and when only adding text to the end of the RenderText.

This patch adds AppendText that allows adding to an exiting RenderText without disturbing the existing styling.

This change is related to (and used by) CL 795933009.

BUG=

Review URL: https://codereview.chromium.org/1014163002

Cr-Commit-Position: refs/heads/master@{#321507}