cros: Add ClipboardHistory UserJourneyTime metric

Add this metric to help us understand usage and plan
improvements.

Bug: 1099338
Change-Id: I77ce043b2e54031b8c35d8ba302be8b7b254edcb
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2424508
Commit-Queue: Alex Newcomer <newcomer@chromium.org>
Reviewed-by: Andrew Xu <andrewxu@chromium.org>
Reviewed-by: Mark Pearson <mpearson@chromium.org>
Cr-Commit-Position: refs/heads/master@{#811925}

tools/metrics/histograms/histograms_xml/ash/histograms.xml

@@ -201,8 +201,8 @@ reviews. Googlers can read more about this at go/gwsq-gerrit.
   <owner>newcomer@chromium.org</owner>
   <summary>
     The length of the user journey of the context menu version of clipboard
-    history. Measured as time before the menu is constructed, to time of the
-    menu being closed. Recorded every time the menu is closed, regardless of
+    history. Measured as time from starting to construct the menu, to time of
+    the menu being closed. Recorded every time the menu is closed, regardless of
     whether the user selected one of the menu options.
   </summary>
 </histogram>