Call isValidProtocol() function first in order to remove duplicated protocol checking in NavigatorContentUtils

isValidProtocol() can check if protocl contains colon. If we call isValidProtocol() first in verifyProtocolHandlerScheme(), we're able to remove unnecessary protocol checking logic.

TEST=navigatorcontentutils
BUG=none

Review URL: https://codereview.chromium.org/349193002

git-svn-id: svn://svn.chromium.org/blink/trunk@176894 bbb929c8-8fbe-4397-9dbb-9b2b20218538