Skip checking type name in conversion operator.

A line starting with optional whitespaces and 'operator' is unlikely a
declaration of a new identifier. The type name after operator is
declared elsewhere. Checking of type name in conversion operator should
be skipped.

BUG=290417

Review URL: https://chromiumcodereview.appspot.com/24124002

git-svn-id: svn://svn.chromium.org/blink/trunk@157730 bbb929c8-8fbe-4397-9dbb-9b2b20218538