Add a histogram definition for Linux.GlibcVersion.

tools/metrics/histograms/histograms.xml

Index: tools/metrics/histograms/histograms.xml
===================================================================
--- tools/metrics/histograms/histograms.xml	(revision 195312)
+++ tools/metrics/histograms/histograms.xml	(working copy)
@@ -493,6 +493,10 @@
   </summary>
 </histogram>
 
+<histogram name="Linux.GlibcVersion" enum="LinuxGlibcVersion">
+  <summary>The version of glibc used. (Linux only)</summary>
+</histogram>
+
 <histogram name="Media.DetectedAudioCodec" enum="FFmpegCodecs">
   <summary>Audio codec used in HTML5 media.</summary>
 </histogram>
@@ -5577,6 +5581,20 @@
   <int value="18" label="NewLogger"/>
 </enum>
 
+<enum name="LinuxGlibcVersion" type="int">
+  <int value="0" label="Not Parseable"/>
+  <int value="1" label="Unknown"/>
+  <int value="2" label="2.11"/>
+  <int value="3" label="2.12"/>
+  <int value="4" label="2.13"/>
+  <int value="5" label="2.14"/>
+  <int value="6" label="2.15"/>
+  <int value="7" label="2.16"/>
+  <int value="8" label="2.17"/>
+  <int value="9" label="2.18"/>
+  <int value="10" label="2.19"/>
+</enum>
+
 <enum name="LoadType" type="int">
   <int value="0" label="UNDEFINED_LOAD">Not yet initialized</int>
   <int value="1" label="RELOAD">User pressed reload</int>