Remove listening of OMNIBOX_OPENED_URL from metrics code

I am working to eliminate this notification entirely, as its usage is impeding
iOS upstreaming (see bug for details). This CL replaces its usage in
//chrome/browser/metrics with usage of OmniboxEventGlobalTracker.

BUG=517885

Review URL: https://codereview.chromium.org/1282473002

Cr-Commit-Position: refs/heads/master@{#343662}
4 files changed