Log impressions of the onboarding CTA.

Log when the user is or isn't show the onboarding CTA using the
GettingStartedRedirectImpression schema, which includes the type of the
CTA, i.e. if the user was prompted to edit the page and if the user was
prompted to find other pages that needed editing.

Change-Id: Ife8a55d153209e8348d806c80b5567024603b308
Merge "Log impressions of the onboarding CTA."