histograms.xml: add forgotten kCodecHEVC in enumeration

This CL adds it because it's in [1] but not in [2]

[1] https://cs.chromium.org/chromium/src/media/base/video_codecs.h?q=kCodecHEVC&sq=package:chromium&l=27&dr=CSs
[2] https://cs.chromium.org/chromium/src/tools/metrics/histograms/histograms.xml?q=histograms.xml+kCodecVP9&sq=package:chromium&dr=C&l=98435

Review-Url: https://codereview.chromium.org/2347793002
Cr-Commit-Position: refs/heads/master@{#419223}

tools/metrics/histograms/histograms.xml

@@ -98476,6 +98476,7 @@ To add a new entry, add it with any value and run test to compute valid value.
   <int value="5" label="kCodecTheora"/>
   <int value="6" label="kCodecVP8"/>
   <int value="7" label="kCodecVP9"/>
+  <int value="8" label="kCodecHEVC"/>
 </enum>
 
 <enum name="VideoCodecProfile" type="int">