Remove useless calls to v8::Value::ToString()

Remove useless calls to v8::Value::ToString() after we already checked
that v8::Value::IsString() already returns true. Use
v8::Handle::As<v8::String>() instead, which is more efficient as it
does not call v8::Value::IsString() again.

R=haraken
BUG=

Review URL: https://chromiumcodereview.appspot.com/23578025

git-svn-id: svn://svn.chromium.org/blink/trunk@157678 bbb929c8-8fbe-4397-9dbb-9b2b20218538