[ChromeDriver] Fix uninitialized variable in test

Unit test Device.StartStopApp is failing on msan builds due to an
uninitialized variable, introduced by crrev.com/806405. Fixing it.

TBR=shengfa@google.com

Bug: chromedriver:2421
Change-Id: I26e5bceb30d3ba82de5c6502f71a5297617240be
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2408884Reviewed-by: John Chen <johnchen@chromium.org>
Commit-Queue: John Chen <johnchen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#806547}

chrome/test/chromedriver/chrome/device_manager_unittest.cc

@@ -156,7 +156,7 @@ TEST(DeviceManager, AcquireSpecificDevice) {
 }
 
 TEST(Device, StartStopApp) {
-  int devtools_port;
+  int devtools_port = 0;
   FakeAdb adb;
   DeviceManager device_manager(&adb);
   std::unique_ptr<Device> device1;