Add UMA histograms for WebUSB.

chrome/browser/usb/usb_chooser_bubble_delegate.cc

Index: chrome/browser/usb/usb_chooser_bubble_delegate.cc
diff --git a/chrome/browser/usb/usb_chooser_bubble_delegate.cc b/chrome/browser/usb/usb_chooser_bubble_delegate.cc
index f8f817c8281a1e114dfadba888c2c5adaf1e3d57..bc10ad96127abf2355a75640fc5029f384083d72 100644
--- a/chrome/browser/usb/usb_chooser_bubble_delegate.cc
+++ b/chrome/browser/usb/usb_chooser_bubble_delegate.cc
@@ -8,6 +8,7 @@
 #include <utility>
 
 #include "base/bind.h"
+#include "base/metrics/histogram_macros.h"
 #include "base/stl_util.h"
 #include "chrome/browser/profiles/profile.h"
 #include "chrome/browser/usb/usb_chooser_context.h"
@@ -23,6 +24,21 @@
 
 namespace {
 
+enum WebUsbChooserClosed {
+  // The user cancelled the permission prompt without selecting a device.
+  WEBUSB_CHOOSER_CLOSED_CANCELLED = 0,
+  // The user probably cancelled the permission prompt without selecting a
+  // device because there were no devices to select.
+  WEBUSB_CHOOSER_CLOSED_CANCELLED_NO_DEVICES,
+  // The user granted permission to access a device.
+  WEBUSB_CHOOSER_CLOSED_PERMISSION_GRANTED,
+  // The user granted permission to access a device but that permission will be
+  // revoked when the device is disconnected.
+  WEBUSB_CHOOSER_CLOSED_EPHEMERAL_PERMISSION_GRANTED,
+  // Maximum value for the enum.
+  WEBUSB_CHOOSER_CLOSED_MAX
+};
+
 // Check if the origin is in the description set.
 bool FindOriginInDescriptorSet(const device::WebUsbDescriptorSet* set,
                                const GURL& origin) {
@@ -105,11 +121,32 @@ void UsbChooserBubbleDelegate::Select(size_t index) {
   callback_.Run(std::move(device_info_ptr));
   callback_.reset();  // Reset |callback_| so that it is only run once.
 
+  if (devices_[index].first->serial_number().empty()) {
+    UMA_HISTOGRAM_ENUMERATION(
+        "WebUsb.ChooserClosed",
+        WEBUSB_CHOOSER_CLOSED_EPHEMERAL_PERMISSION_GRANTED,

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

Nit: Each histogram macro expands to a lot of code. Can you make a helper
function for this that just takes the enum as a param?

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

Done.

+        WEBUSB_CHOOSER_CLOSED_MAX);
+  } else {
+    UMA_HISTOGRAM_ENUMERATION("WebUsb.ChooserClosed",
+                              WEBUSB_CHOOSER_CLOSED_PERMISSION_GRANTED,
+                              WEBUSB_CHOOSER_CLOSED_MAX);
+  }
+
   if (bubble_controller_)
     bubble_controller_->CloseBubble(BUBBLE_CLOSE_ACCEPTED);
 }
 
 void UsbChooserBubbleDelegate::Cancel() {
+  if (devices_.size() == 0) {
+    UMA_HISTOGRAM_ENUMERATION("WebUsb.ChooserClosed",
+                              WEBUSB_CHOOSER_CLOSED_CANCELLED_NO_DEVICES,
+                              WEBUSB_CHOOSER_CLOSED_MAX);
+  } else {
+    UMA_HISTOGRAM_ENUMERATION("WebUsb.ChooserClosed",
+                              WEBUSB_CHOOSER_CLOSED_CANCELLED,
+                              WEBUSB_CHOOSER_CLOSED_MAX);
+  }
+
   if (bubble_controller_)
     bubble_controller_->CloseBubble(BUBBLE_CLOSE_CANCELED);
 }