In VideoCaptureV4L2 create the capture thread last in StartCapture

This makes it possible to add a SequenceChecker guard to _deviceFd that
ensures it is accessed only on the api thread while the capture thread
is not running, and only on the capture thread otherwise.

Bug: webrtc:15181
Change-Id: Ibc414ee973a3c4798e38e9b9a63e3053b95b9599
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/305645
Reviewed-by: Per Kjellander <perkj@webrtc.org>
Reviewed-by: Ilya Nikolaevskiy <ilnik@webrtc.org>
Commit-Queue: Ilya Nikolaevskiy <ilnik@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#40194}

Author: Pehrsons

modules/video_capture/linux/video_capture_v4l2.cc

@@ -268,6 +268,16 @@ int32_t VideoCaptureModuleV4L2::StartCapture(
     return -1;
   }
 
+  // Needed to start UVC camera - from the uvcview application
+  enum v4l2_buf_type type;
+  type = V4L2_BUF_TYPE_VIDEO_CAPTURE;
+  if (ioctl(_deviceFd, VIDIOC_STREAMON, &type) == -1) {
+    RTC_LOG(LS_INFO) << "Failed to turn on stream";
+    return -1;
+  }
+
+  _captureStarted = true;
+
   // start capture thread;
   if (_captureThread.empty()) {
     quit_ = false;
@@ -279,16 +289,6 @@ int32_t VideoCaptureModuleV4L2::StartCapture(
         "CaptureThread",
         rtc::ThreadAttributes().SetPriority(rtc::ThreadPriority::kHigh));
   }
-
-  // Needed to start UVC camera - from the uvcview application
-  enum v4l2_buf_type type;
-  type = V4L2_BUF_TYPE_VIDEO_CAPTURE;
-  if (ioctl(_deviceFd, VIDIOC_STREAMON, &type) == -1) {
-    RTC_LOG(LS_INFO) << "Failed to turn on stream";
-    return -1;
-  }
-
-  _captureStarted = true;
   return 0;
 }