Get rid of last uses of deprecated toWebCoreString(v8::Handle<v8::Value>)

Get rid of last uses of deprecated toWebCoreString(v8::Handle<v8::Value>). Calls
have either been replaced by the new toWebCoreString(v8::String) or by the
V8TRYCATCH_FOR_V8STRINGRESOURCE*() macros in cases where it can throw an
exception.

R=haraken

Review URL: https://codereview.chromium.org/106983007

git-svn-id: svn://svn.chromium.org/blink/trunk@163488 bbb929c8-8fbe-4397-9dbb-9b2b20218538