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Since HTTPS Server Previews will also send pingbacks, and we don't want
a lot of copied code, this factors all DRP pingback logic out into a
shareable base class. The histogram recording stays in the DRP Observer.

Also factors out a test_utils class for testing pingbacks.

This CL has no behavorial changes to pingbacks themselves. However, in
the next CL, the OnCommit logic will move from the base class to the
derived classes.

Bug: 864665
Change-Id: I569bc58331b45b71ae702149e43a2630d523e6dc
Reviewed-on: https://chromium-review.googlesource.com/c/1351497
Commit-Queue: Robert Ogden <robertogden@chromium.org>
Reviewed-by: Ryan Sturm <ryansturm@chromium.org>
Cr-Commit-Position: refs/heads/master@{#611364}