Add UMA histograms for WebUSB.

chrome/browser/usb/usb_chooser_context.cc

Index: chrome/browser/usb/usb_chooser_context.cc
diff --git a/chrome/browser/usb/usb_chooser_context.cc b/chrome/browser/usb/usb_chooser_context.cc
index 702c98adc162caa1287464026dd3efd3a40b3f37..fd1862c2fead199f85633bce1da9b01268c3c40c 100644
--- a/chrome/browser/usb/usb_chooser_context.cc
+++ b/chrome/browser/usb/usb_chooser_context.cc
@@ -7,6 +7,7 @@
 #include <utility>
 #include <vector>
 
+#include "base/metrics/histogram_macros.h"
 #include "base/stl_util.h"
 #include "base/strings/utf_string_conversions.h"
 #include "base/values.h"
@@ -24,6 +25,15 @@ const char kProductIdKey[] = "product-id";
 const char kSerialNumberKey[] = "serial-number";
 const char kVendorIdKey[] = "vendor-id";
 
+enum WebUsbPermissionRevoked {
+  // Permission to access a USB device was revoked by the user.
+  WEBUSB_PERMISSION_REVOKED = 0,
+  // Permission to access an ephemeral USB device was revoked by the user.
+  WEBUSB_PERMISSION_REVOKED_EPHEMERAL,
+  // Maximum value for the enum.
+  WEBUSB_PERMISSION_REVOKED_MAX
+};
+
 bool CanStorePersistentEntry(const scoped_refptr<const UsbDevice>& device) {
   return !device->serial_number().empty();
 }
@@ -116,9 +126,15 @@ void UsbChooserContext::RevokeObjectPermission(
   std::string guid;
   if (object.GetString(kGuidKey, &guid)) {
     RevokeDevicePermission(requesting_origin, embedding_origin, guid);
+    UMA_HISTOGRAM_ENUMERATION("WebUsb.PermissionRevoked",
+                              WEBUSB_PERMISSION_REVOKED_EPHEMERAL,

[Alexei Svitkine (slow), 2016/01/18 20:05:13]

Ditto.

[Reilly Grant (use Gerrit), 2016/01/20 00:59:37]

Done.

+                              WEBUSB_PERMISSION_REVOKED_MAX);
   } else {
     ChooserContextBase::RevokeObjectPermission(requesting_origin,
                                                embedding_origin, object);
+    UMA_HISTOGRAM_ENUMERATION("WebUsb.PermissionRevoked",
+                              WEBUSB_PERMISSION_REVOKED,
+                              WEBUSB_PERMISSION_REVOKED_MAX);
   }
 }