Cleanup string conversions in WebFrameTest.

WebString::utf8 now returns a std::string, so there's no need to construct a
new one from the pointer.

BUG=none

Review URL: https://codereview.chromium.org/26994006

git-svn-id: svn://svn.chromium.org/blink/trunk@159502 bbb929c8-8fbe-4397-9dbb-9b2b20218538