Set correct enum type for metric RequestedPixelFormat.

Was set to the wrong value in https://chromium-review.googlesource.com/c/chromium/src/+/2621817

Bug: 1156144
Change-Id: I82ddd9432ca6aa10ad9ab01edd0cae2fdc45610c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2637578
Commit-Queue: Evan Shrubsole <eshr@google.com>
Reviewed-by: Caitlin Fischer <caitlinfischer@google.com>
Cr-Commit-Position: refs/heads/master@{#844792}

tools/metrics/histograms/histograms_xml/media/histograms.xml

@@ -3913,7 +3913,7 @@ reviews. Googlers can read more about this at go/gwsq-gerrit.
 </histogram>
 
 <histogram name="Media.VideoCapture.Mac.Device.RequestedPixelFormat"
-    enum="VideoResolutionDesignation" expires_after="M92">
+    enum="VideoPixelFormatUnion" expires_after="M92">
   <owner>eshr@google.com</owner>
   <owner>handellm@google.com</owner>
   <summary>