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This CL adds a new metric that records the number back navigations to
a Google search result page in a search session. This is part of efforts
to record metrics for CSN stage 1.

See go/csn-design for more details on the project.

Change-Id: Ida5ac7358764a8c13da5aea8890957f77c6e2a47
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2616325Reviewed-by: Caitlin Fischer <caitlinfischer@google.com>
Reviewed-by: Yaron Friedman <yfriedman@chromium.org>
Reviewed-by: Calder Kitagawa <ckitagawa@chromium.org>
Reviewed-by: Fred Mello <fredmello@chromium.org>
Reviewed-by: Mehran Mahmoudi <mahmoudi@chromium.org>
Commit-Queue: Yashar Dabiran <yashard@chromium.org>
Cr-Commit-Position: refs/heads/master@{#845675}