go.talks/present/playground.js: make text function work with Firefox
go.talks/present/playground.js: make text function work with Firefox

Firefox does not support innerText, instead, it supports the textContent
property.

R=adg
CC=golang-dev
http://codereview.appspot.com/6532051

Committer: Andrew Gerrand <395a7d33bec8475c9b83b7d440f141bcbd994aa5@golang.org>

webfd/go-zh.talks,webfd/go-zh.talks,webfd/go-zh.talks,bryanxu/go-zh.talks,d0f/go-zh.talks,d0f/go-zh.talks,d0f/go-zh.talks,d0f/go-zh.talks,bryanxu/go-zh.talks,webfd/go-zh.talks,webfd/go-zh.talks,bryanxu/go-zh.talks,webfd/go-zh.talks,bryanxu/go-zh.talks,d0f/go-zh.talks,d0f/go-zh.talks,bryanxu/go-zh.talks,bryanxu/go-zh.talks