KURL::isBlankURL should not include about:srcdoc

Rename isBlankURL to isProtocolAbout to match isPrococolData
and hopefully make callers less confused.

This is in preparation for making about:srcdoc not go
through the main-thread parser.

BUG=309862
R=abarth@chromium.org

Review URL: https://codereview.chromium.org/245413002

git-svn-id: svn://svn.chromium.org/blink/trunk@172069 bbb929c8-8fbe-4397-9dbb-9b2b20218538