Remove ASSERT(renderer()) in HTMLTextAreaElement::updateValue()

This was first removed in r181422 and then brought back due to
revert at r182379 and now causes fast/ime/composition-offsets.html
and fast/forms/autofilled.html to crash. It does not look like
updateValue() needs renderer now (it did back when the ASSERT()
was added), so removing the assert again.

TBR=sudarshan.p@samsung.com,keishi@chromium.org,tkent@chromium.org

Review URL: https://codereview.chromium.org/589013002

git-svn-id: svn://svn.chromium.org/blink/trunk@182395 bbb929c8-8fbe-4397-9dbb-9b2b20218538