media: support devices without built-in camera

Some devices like VM or chromeboxes do not have built-in camera at all,
we should not treat it as an error.

BUG=b:77833131,b:64996728,chromium:832816,chromium:852302
TEST=external camera works on teemo (a chromebox)

Change-Id: I2a19afa72ab9e0828f4c65a7761cd2b54d1ecfdd
Reviewed-on: https://chromium-review.googlesource.com/1136451Reviewed-by: Ricky Liang <jcliang@chromium.org>
Commit-Queue: Shik Chen <shik@chromium.org>
Cr-Commit-Position: refs/heads/master@{#574884}

media/capture/video/chromeos/camera_hal_delegate.cc

@@ -287,7 +287,7 @@ void CameraHalDelegate::UpdateBuiltInCameraInfoOnIpcThread() {
 
 void CameraHalDelegate::OnGotNumberOfCamerasOnIpcThread(int32_t num_cameras) {
   DCHECK(ipc_task_runner_->BelongsToCurrentThread());
-  if (num_cameras <= 0) {
+  if (num_cameras < 0) {
     builtin_camera_info_updated_.Signal();
     LOG(ERROR) << "Failed to get number of cameras: " << num_cameras;
     return;
@@ -315,6 +315,12 @@ void CameraHalDelegate::OnSetCallbacksOnIpcThread(int32_t result) {
                << base::safe_strerror(-result);
     return;
   }
+
+  if (num_builtin_cameras_ == 0) {
+    builtin_camera_info_updated_.Signal();
+    return;
+  }
+
   for (size_t camera_id = 0; camera_id < num_builtin_cameras_; ++camera_id) {
     GetCameraInfoOnIpcThread(
         camera_id,