Add Tab.Close / Tab.Close.UnloadTime to histograms.xml.
Add Tab.Close / Tab.Close.UnloadTime to histograms.xml.

The actual histograms were added in
https://chromiumcodereview.appspot.com/17571018

BUG=

Review URL: https://codereview.chromium.org/1476073003

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#370406}

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