Add SiteIsolation.IsolateExtensions metrics and tests.

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 f9b9ebdebef0cf7a01fe6844262ce007491af345..d52587302ba50693d5d5783c886833aee6500c26 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -43371,6 +43371,39 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   </summary>
 </histogram>
 
+<histogram name="SiteIsolation.IsolateExtensionsProcessCountEstimate">

[Ilya Sherman, 2015/09/11 20:08:23]

nit: Maybe name this "UpperBound", to match "LowerBound" below?

[ncarter (slow), 2015/09/11 20:11:09]

This is named Estimate to match the naming scheme of the other histograms in the
SiteIsolation category (e.g. SiteIsolation.IsolateHttpsSitesProcessCountEstimate
and others above/below this diff hunk).

+  <owner>nick@chromium.org</owner>
+  <summary>
+    The upper bound of the predicted renderer process count if we isolated only
+    Chrome extensions, subject to the process limit.  Recorded once per UMA
+    ping.
+  </summary>
+</histogram>
+
+<histogram name="SiteIsolation.IsolateExtensionsProcessCountLowerBound">
+  <owner>nick@chromium.org</owner>
+  <summary>
+    The lower bound of the predicted renderer process count if we isolated only
+    Chrome extensions, subject to the process limit. Recorded once per UMA ping.
+  </summary>
+</histogram>
+
+<histogram name="SiteIsolation.IsolateExtensionsProcessCountNoLimit">
+  <owner>nick@chromium.org</owner>
+  <summary>
+    The predicted renderer process count if we isolated only Chrome extensions
+    and if there were no process limit.  Recorded once per UMA ping.
+  </summary>
+</histogram>
+
+<histogram name="SiteIsolation.IsolateExtensionsTotalProcessCountEstimate">
+  <owner>nick@chromium.org</owner>
+  <summary>
+    The predicted total process count if we isolated only Chrome extensions,
+    subject to the process limit.  Recorded once per UMA ping.
+  </summary>
+</histogram>
+
 <histogram name="SiteIsolation.IsolateHttpsSitesProcessCountEstimate">
   <owner>creis@chromium.org</owner>
   <summary>