Make metadata state local in the WebVTT parser

The members m_currentHeaderName and m_currentHeaderValue are only used
within the scope of WebVTTParser::collectMetadataHeader, so there's no
need to keep them in instance scope. Make them local to said method,
and pass the value explicitly to its users.

BUG=305317

Review URL: https://codereview.chromium.org/65343003

git-svn-id: svn://svn.chromium.org/blink/trunk@161922 bbb929c8-8fbe-4397-9dbb-9b2b20218538