Rename 'ScalarValueString' to 'USVString' to match Web IDL

Originally proposed in the Encoding standard as 'ScalarValueString',
this type was finally added to Web IDL with the name 'USVString'
("Unicode scalar value string"). The behavior is identical: unpaired
UTF-16 surrogate code units in JS strings are converted to U+FFFD
replacement characters by the binding layer.

The type name is not exposed to script, so this is not a web-exposed
change.

Spec: http://heycam.github.io/webidl/#idl-USVString

BUG=423660
R=haraken@chromium.org

Review URL: https://codereview.chromium.org/644473006

git-svn-id: svn://svn.chromium.org/blink/trunk@184312 bbb929c8-8fbe-4397-9dbb-9b2b20218538