Handling radio group name as case insensitive

Added CaseFoldingHash in the NameToGroupMap to handle group name case
insensitive.
(Replicates patch fixed in WebKit for this issue:
https://bugs.webkit.org/attachment.cgi?id=187180)

Note that some test cases in radio-group-name-case.html fails because
CaseFoldingHash doesn't conform to the specification.

R=tkent
BUG=136105
TEST=automated

Review URL: https://codereview.chromium.org/149173007

git-svn-id: svn://svn.chromium.org/blink/trunk@166241 bbb929c8-8fbe-4397-9dbb-9b2b20218538