Further WTF::copyChars simplification

This patch removes the code branch handling "numCharacters == 1"
case leaving only the 'memcpy' call inside the 'WTF::copyChars'
function's body.

The reason for it is that the branch made the code more complex
giving the minor performance benefit: there was no benefit if the
'source' arg was static, otherwise the difference was just two
extra asm calls ('testq' and 'je') which should not give impact
to the real use cases performance (tested with GCC 4.6.3).

Review URL: https://codereview.chromium.org/213073002

git-svn-id: svn://svn.chromium.org/blink/trunk@170299 bbb929c8-8fbe-4397-9dbb-9b2b20218538