Improve RTCIceCandidate exception messages.

Added a more helpful exception message when no candidate string
is provided to the RTCIceCandidate constructor, and adjusts the layout
test accordingly.

Also corrected one test case which contained a syntax error to instead
exercise the constructor.

BUG=270033

Review URL: https://codereview.chromium.org/156593003

git-svn-id: svn://svn.chromium.org/blink/trunk@166851 bbb929c8-8fbe-4397-9dbb-9b2b20218538