[CrOS PhoneHub] Metrics file comment fix "WebLayer" -> "PhoneHub".

Bug: 1106938
Change-Id: Ifbada6d7ef9552ffc0e7ee3a3429f30422c1eacf
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2498838Reviewed-by: Weilun Shi <sweilun@chromium.org>
Reviewed-by: Steven Holte <holte@chromium.org>
Commit-Queue: Regan Hsu <hsuregan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#822235}

tools/metrics/histograms/histograms_xml/phonehub/histograms.xml

@@ -5,7 +5,7 @@ found in the LICENSE file.
 -->
 
 <!--
-This file is used to generate a comprehensive list of WebLayer histograms
+This file is used to generate a comprehensive list of PhoneHub histograms
 along with a detailed description for each histogram.
 
 For best practices on writing histogram descriptions, see