Refactor verifyProtocolHandlerScheme() in NavigatorContentUtils.cpp

Remove redundant parameter in this function as well as rename the function name
to verifyCustomHandlerScheme() to be sync with other helper function.

BUG=none

Review URL: https://codereview.chromium.org/398743002

git-svn-id: svn://svn.chromium.org/blink/trunk@178264 bbb929c8-8fbe-4397-9dbb-9b2b20218538