Initialize urb to make MSan happy.

Bug: 1136327
Change-Id: Id47e06bfd088e5c72d5f41c091882e99768fb93d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2459447
Commit-Queue: Reilly Grant <reillyg@chromium.org>
Auto-Submit: David Manouchehri <david@davidmanouchehri.com>
Reviewed-by: Reilly Grant <reillyg@chromium.org>
Cr-Commit-Position: refs/heads/master@{#815025}

services/device/usb/usb_device_handle_usbfs.cc

@@ -356,7 +356,7 @@ void UsbDeviceHandleUsbfs::BlockingTaskRunnerHelper::
   for (size_t i = 0; i < MAX_URBS_PER_EVENT; ++i) {
     base::ScopedBlockingCall scoped_blocking_call(
         FROM_HERE, base::BlockingType::MAY_BLOCK);
-    usbdevfs_urb* urb;
+    usbdevfs_urb* urb = nullptr;
     int rc = HANDLE_EINTR(ioctl(fd_.get(), USBDEVFS_REAPURBNDELAY, &urb));
     if (rc) {
       if (errno == EAGAIN)