Telemetry: Start clank with blank page.

Instead of starting clank with the NewTabPage, open a blank page. The current
NTP triggers some URL changes which interfere with navigation to a page if they
happen simultaneously. In these cases the page that we want to navigate to
isn't being loaded.

BUG=238864

Review URL: https://codereview.chromium.org/27320002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@228724 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 228724