Translate: adopt html lang attribute if valid value is provided

tools/metrics/histograms/histograms.xml

Index: tools/metrics/histograms/histograms.xml
diff --git a/tools/metrics/histograms/histograms.xml b/tools/metrics/histograms/histograms.xml
index 39d1daf36b0bd681dbec39a238fc11825401ba96..df0bf271ae74e6db71323f86097bbb34a611cfb6 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -8950,13 +8950,20 @@ other types of suffix sets.
   </summary>
 </histogram>
 
-<histogram name="Translate.ContentLanguage" enum="TranslateContentLanguage">
+<histogram name="Translate.ContentLanguage" enum="TranslateLanguage">
   <summary>
     A page may provide a Content-Language HTTP header or a META tag. For each
     page load, measures whether the Content-Language header exists and is valid.
   </summary>
 </histogram>
 
+<histogram name="Translate.HtmlLang" enum="TranslateLanguage">
+  <summary>
+    A page may provide a lang attribute in html tag. For each page load,
+    measures whether the lang attribute exists and is valid.
+  </summary>
+</histogram>
+
 <histogram name="Translate.InitiationStatus" enum="TranslateInitiationStatus">
   <summary>
     The reason why Chrome decided to perform the next action (e.g., to show
@@ -14073,12 +14080,6 @@ other types of suffix sets.
   </int>
 </enum>
 
-<enum name="TranslateContentLanguage" type="int">
-  <int value="0" label="No Content-Language"/>
-  <int value="1" label="Valid Content-Language"/>
-  <int value="2" label="Invalid Content-Language"/>
-</enum>
-
 <enum name="TranslateError" type="int">
   <int value="0" label="No error"/>
   <int value="1" label="Network error"/>
@@ -14102,6 +14103,12 @@ other types of suffix sets.
   <int value="9" label="Show infobar"/>
 </enum>
 
+<enum name="TranslateLanguage" type="int">
+  <int value="0" label="No language code"/>
+  <int value="1" label="Valid language code"/>
+  <int value="2" label="Invalid language code"/>
+</enum>
+
 <enum name="TranslateLanguageVerification" type="int">
   <int value="0" label="CLD is disabled"/>
   <int value="1" label="No Content-Language"/>