Remove unecessary malloc seen in http://sina.com.cn (#17)

Content calls down into Blink a bunch of times during loading to
check the document's encoding.  Every time we were allocating a
new copy of a constant string.  The right fix is to fix TextEncoding
to have fancier static-string technology, but this is the quick-fix
to get this malloc off the profile of http://sina.com.cn.

BUG=261689

Review URL: https://chromiumcodereview.appspot.com/19663003

git-svn-id: svn://svn.chromium.org/blink/trunk@154508 bbb929c8-8fbe-4397-9dbb-9b2b20218538