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A previous CL https://crrev.com/748972 records metrics for toggling
two-up view mode in the PDF viewer.

This is a follow-up CL, which adds the enumerations used for two-up
view actions' metrics.

Bug: 51472
Change-Id: Iae03eaea36d07ebdbe4eb52df5e50709b2e13d1c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2097495
Commit-Queue: Hui Yingst <nigi@chromium.org>
Reviewed-by: Lei Zhang <thestig@chromium.org>
Cr-Commit-Position: refs/heads/master@{#749165}