Shrink isValidNameNonASCII by using U16_NEXT only once.

This function was much larger than expected (314 bytes in gcc) and I think
it is because U16_NEXT is a large macro. Use it once instead of
twice.

Seems to have saved 144 bytes with gcc. U16_NEXT must be a crazily large macro:

  Shrunk symbols:
       -144: WebCore::Document::isValidName(WTF::String const&) type=t, (was 1253 bytes, now 1109 bytes)

74 bytes saved with clang:
 Shrunk symbols:
        -74: WebCore::Document::isValidName(WTF::String const&) type=t, (was 1190 bytes, now 1116 bytes)


BUG=

Review URL: https://codereview.chromium.org/387313002

git-svn-id: svn://svn.chromium.org/blink/trunk@182236 bbb929c8-8fbe-4397-9dbb-9b2b20218538