[Telemetry] Ignore white in Speed Index calculation (like WPT does).

BUG=323813
R=slamm@chromium.org, szym@chromium.org

Review URL: https://codereview.chromium.org/136213002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@244712 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 244712