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 ba8bf977d74849dcaf4ae68e7a3e32354bbb3ebc..b255ee94e7fd49bf6ce70a6979bbaad84e038ebb 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -10181,6 +10181,30 @@ 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>
+    Recorded only for Android. How a speculation was started or why it was
+    disallowed. This is recorded when a speculation of any kind is requested,
+    with either a &quot;Speculation allowed&quot; value, or an aborted value. If

[Mark P, 2017/06/06 18:25:50]

nit: "aborted", in my understanding of English, typically means started but
cancelled at some point.  As I understand this histogram, the value for
"aborted" mean "the reason this was not started".  Can you instead use a word
such as "prohibited" or "prevented" that doesn't imply there it was originally
"started"?

(Also, revise the enums accordingly.)

[mattcary, 2017/06/07 11:55:47]

Done.

+    the speculation is allowed and is attempted to be started, the status on
+    start will be counted in addition: one of "Prefetch",
+    "Prerender", "Background Tab" or "Prerender not
+    started". The latter case is when a prerender is abandoned during

[Mark P, 2017/06/06 18:25:50]

nit: latter -> last

[mattcary, 2017/06/07 11:55:47]

Done.

+    start.
+  </summary>
+</histogram>
+
+<histogram name="CustomTabs.SpeculationStatusOnSwap"
+    enum="CustomTabsSpeculationStatusOnSwap">
+  <owner>mattcary@chromium.org</owner>
+  <summary>
+    Recorded only for Android. Recorded when a speculation is requesting to swap
+    into a visible tab.
+  </summary>
+</histogram>
+
 <histogram name="CustomTabs.Visible" enum="VisibleTab">
   <owner>yusufo@chromium.org</owner>
   <summary>