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friends) to measure the time from the URL starting to load to it loading.
Extensions.BackgroundPageLoadTime2 replaces Extensions.BackgroundPageLoadTime
etc.

Previously it measured from ExtensionHost construction to it loading, which is
wrong because URL loading can be deferred in a queue, or callers just might not
try to load it until later.

Also do a bit of UMA cleanup: delete some of the Extension page load UMA that
isn't used anymore, and add back the Extensions.PopupLoadTime2 load UMA which
was forgotten about at some point.

R=yoz@chromium.org, isherman@chromium.org
BUG=453073

Review URL: https://codereview.chromium.org/902963002

Cr-Commit-Position: refs/heads/master@{#315149}