Add UMA metrics for the linux distro.

tools/metrics/histograms/enums.xml

Index: tools/metrics/histograms/enums.xml
diff --git a/tools/metrics/histograms/enums.xml b/tools/metrics/histograms/enums.xml
index 82f9bf1eeadea1b500bfac3d92ba14d1ae405b03..b56827669cddc0ba3ab039fcf21f875b64ce324c 100644
--- a/tools/metrics/histograms/enums.xml
+++ b/tools/metrics/histograms/enums.xml
@@ -20965,6 +20965,22 @@ uploading your change for review.  These are checked by presubmit scripts.
   <int value="20" label="OTHER_LIBSECRET"/>
 </enum>
 
+<enum name="LinuxDistro" type="int">
+  <int value="0" label="Unknown"/>
+  <int value="1" label="Ubuntu Other"/>
+  <int value="2" label="Ubuntu 14.04"/>
+  <int value="3" label="Ubuntu 16.04"/>
+  <int value="4" label="Ubuntu 16.10"/>
+  <int value="5" label="Ubuntu 17.04"/>
+  <int value="6" label="Debian Other"/>
+  <int value="7" label="Debian 8"/>
+  <int value="8" label="openSUSE Other"/>
+  <int value="9" label="openSUSE Leap 42.2"/>
+  <int value="10" label="Fedora Other"/>
+  <int value="11" label="Fedora 24"/>
+  <int value="12" label="Fedora 25"/>
+</enum>
+
 <enum name="LinuxGlibcVersion" type="int">
   <int value="0" label="Not Parseable"/>
   <int value="1" label="Unknown"/>