[telemetry] Clear cache before navigating in record_wpr.

This ensures all resources are fetched from network.

Review URL: https://codereview.chromium.org/23531086

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@225997 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 225997