Switch to the correct Blink type, UChar.

The use of char16 was inadvertently pulling in Chromium's base/strings/string16.h, which isn't what we want.

BUG=329295
TEST=no change

Review URL: https://codereview.chromium.org/121133002

git-svn-id: svn://svn.chromium.org/blink/trunk@164336 bbb929c8-8fbe-4397-9dbb-9b2b20218538