Fix coverage report in webkit/blink based browsers

In webkit, text inputs are limited to 512k
<https://bugs.webkit.org/show_bug.cgi?id=44883>. This replaces it with
a textarea that has no such limitation.

Note that opera 12 has the same limitation on <input> and <textarea>.
As it is replaced by opera 15 which is blink based, it should not be such
a problem.