Add a new LaunchMode value for launches from a user experiment.

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 f258ef1452aee155f317c97c5aa4ccb84e3bb067..128c96455ef5436cfda9695d840790ede1385372 100644
--- a/tools/metrics/histograms/enums.xml
+++ b/tools/metrics/histograms/enums.xml
@@ -20818,6 +20818,32 @@ uploading your change for review.  These are checked by presubmit scripts.
   <int value="268959744" label="NEW_TASK | NEW_DOCUMENT"/>
 </enum>
 
+<enum name="LaunchMode" type="int">
+  <int value="1" label="LM_AS_WEBAPP">
+    Launched as an installed web application
+  </int>
+  <int value="2" label="LM_WITH_URLS">Launched with urls in the cmd line</int>
+  <int value="3" label="LM_SHORTCUT_NONE">Not launched from a shortcut</int>

[Peter Kasting, 2017/05/26 00:57:41]

Is this mutually exclusive with the other items that aren't LM_SHORTCUT_XXX? 
Because the description here doesn't really imply that.

[grt (UTC plus 2), 2017/05/29 07:08:42]

Done.

+  <int value="4" label="LM_SHORTCUT_NONAME">
+    Launched from shortcut but no name available

[Peter Kasting, 2017/05/26 00:57:41]

"Name" in what sense?

[grt (UTC plus 2), 2017/05/29 07:08:42]

Done.

+  </int>
+  <int value="5" label="LM_SHORTCUT_UNKNOWN">
+    Launched from user-defined shortcut
+  </int>
+  <int value="6" label="LM_SHORTCUT_QUICKLAUNCH">
+    Launched from the quick launch bar (pre-Win7)

[Peter Kasting, 2017/05/26 00:57:42]

Seems like this value must be deprecated since we don't support Win 7 anymore.

[grt (UTC plus 2), 2017/05/29 07:08:42]

We still create the QL shortcut for all installs and still have data arriving
from XP/Vista installs. It's difficult but not impossible to use the QL shortcut
on Win7+ machines if one were to follow one of the online guides for
resurrecting the QL area of the taskbar. I'm inclined to leave this as-is until
such time as the code is removed. WDYT?

[Peter Kasting, 2017/05/31 01:54:02]

OK.

+  </int>
+  <int value="7" label="LM_SHORTCUT_DESKTOP">
+    Launched from a desktop shortcut
+  </int>
+  <int value="8" label="LM_SHORTCUT_TASKBAR">
+    Launched from the taskbar (Win7 and higher)
+  </int>
+  <int value="9" label="LM_USER_EXPERIMENT">
+    Launched from the installer as part of a retention experiment
+  </int>
+</enum>
+
 <enum name="LazyCSSParseUsage" type="int">
   <int value="0" label="&gt;= 0%"/>
   <int value="1" label="&gt; 10%"/>