[Extensions] Make port creation synchronous in unload handlers

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 21dfce511ef8ecf7e954ddd98172774aa3722bba..b75134858a4367606bcb16f9ffd1f96f6153aa4b 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -16345,6 +16345,18 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   <summary>The manifest version of each loaded extension.</summary>
 </histogram>
 
+<histogram name="Extensions.Messaging.ExtensionPortsCreated"
+    units="number of ports">
+  <owner>rdevlin.cronin@chromium.org</owner>
+  <summary>
+    The number of ports created in a given script context at a specific time.
+    Recorded once at the destruction of the script context (when no more ports
+    could be created) if and only if at least one port was created in the
+    context. Only includes ports to extension contexts (i.e., not ports created
+    to tabs or native apps).
+  </summary>
+</histogram>
+
 <histogram name="Extensions.Messaging.GetPortIdAsyncTime" units="ms">
   <owner>rdevlin.cronin@chromium.org</owner>
   <summary>
@@ -104534,6 +104546,17 @@ value.
   <affected-histogram name="Extensions.Functions.SucceededTime"/>
 </histogram_suffixes>
 
+<histogram_suffixes name="ExtensionMessagingPortCreationTime" separator=".">
+  <suffix name="Normal"
+      label="Created during any time other than the 'unload' or
+             'beforeunload' handlers."/>
+  <suffix name="InBeforeUnload"
+      label="Created during an event handler for the 'beforeunload' event."/>
+  <suffix name="InUnload"
+      label="Created during an event handler for the 'unload' event."/>
+  <affected-histogram name="Extensions.Messaging.ExtensionPortsCreated"/>
+</histogram_suffixes>
+
 <histogram_suffixes name="ExtensionMessagingPortType" separator=".">
   <suffix name="Extension" label="A port opened to an extension context."/>
   <suffix name="NativeApp" label="A port opened to a native application."/>