Add enumerations for two-up view actions' metrics data.

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}

tools/metrics/histograms/enums.xml

@@ -8838,6 +8838,10 @@ histogram as enum -->
   <int value="32" label="AnnotateStrokeDeviceMouse"/>
   <int value="33" label="AnnotateStrokeDevicePenFirst"/>
   <int value="34" label="AnnotateStrokeDevicePen"/>
+  <int value="35" label="TwoUpViewEnableFirst"/>
+  <int value="36" label="TwoUpViewEnable"/>
+  <int value="37" label="TwoUpViewDisableFirst"/>
+  <int value="38" label="TwoUpViewDisable"/>
 </enum>
 
 <enum name="ChromePDFViewerAnnotationType">