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Add a new print preview event and log when print preview is initiated.

Bug: b:155084154
Change-Id: I400a97277b5c6d54c0221e52792b4e863feb81a6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2168191
Commit-Queue: Jesse Schettler <jschettler@chromium.org>
Reviewed-by: Lei Zhang <thestig@chromium.org>
Cr-Commit-Position: refs/heads/master@{#763521}