Custom Tabs: add speculation status histograms.

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 7cfb8759a2bb05d1c6d81279a475e0b4178f527e..4c3e59966c9509ddfc08e7b57a478d068a2b5edc 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -9807,6 +9807,22 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   </summary>
 </histogram>
 
+<histogram name="CustomTabs.SpeculationStatusOnStart"
+    enum="CustomTabsSpeculationStatusOnStart">
+  <owner>mattcary@chromium.org</owner>
+  <summary>
+    Android: How a speculation was started or why it was aborted.

[pasko, 2017/05/16 14:35:50]

CustomTabs is Android-only, so there is no extra bit of information.

We should document it more verbosely, as this text will be seen on the
dashboard. I've left a few length increases in enums.xml. Here we should mention
something like:

"Recorded when a loading speculation is requested from the CustomTabs service."
and also a few words about "Allowed" being a sum of X, Y and Z.

[mattcary, 2017/05/22 10:07:11]

The other histograms mention android, so I was being consistent. Also, not
everyone may know that custom tabs implies android, so it seems useful even if
it's redundant.
Done.

+  </summary>
+</histogram>
+
+<histogram name="CustomTabs.SpeculationStatusOnSwap"
+    enum="CustomTabsSpeculationStatusOnSwap">
+  <owner>mattcary@chromium.org</owner>
+  <summary>
+    Android: What happened when a speculation was attempted to be swapped.

[pasko, 2017/05/16 14:35:50]

Same as above, please write explicitly "Recorded when ..."

[mattcary, 2017/05/22 10:07:11]

Done.

+  </summary>
+</histogram>
+
 <histogram name="CustomTabs.Visible" enum="VisibleTab">
   <owner>yusufo@chromium.org</owner>
   <summary>