Remove unnecessary DCHECK in stream buffer manager

SetUpStreamsAndBuffers may be called twice when enabling blob stream.
For the second call, kPreview stream already exists.
Also reset repeating_request_settings_ in StopPreview.

BUG=chromium:900900
TEST=./capture_unittest --gtest_filter="*VideoCaptureDeviceTests/*"
Pass TakePhoto cases with DCHECK enabled.

Change-Id: Iac89964b95e7d14f592c779f11a92ef46dba930e
Reviewed-on: https://chromium-review.googlesource.com/c/1332974Reviewed-by: Ricky Liang <jcliang@chromium.org>
Commit-Queue: Heng-ruey Hsu <henryhsu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#607943}

media/capture/video/chromeos/stream_buffer_manager.cc

@@ -81,7 +81,6 @@ void StreamBufferManager::SetUpStreamsAndBuffers(
     const cros::mojom::CameraMetadataPtr& static_metadata,
     std::vector<cros::mojom::Camera3StreamPtr> streams) {
   DCHECK(ipc_task_runner_->BelongsToCurrentThread());
-  DCHECK(!stream_context_[StreamType::kPreview]);
 
   // The partial result count metadata is optional; defaults to 1 in case it
   // is not set in the static metadata.
@@ -191,6 +190,7 @@ void StreamBufferManager::StopPreview(
     base::OnceCallback<void(int32_t)> callback) {
   DCHECK(ipc_task_runner_->BelongsToCurrentThread());
   capturing_ = false;
+  repeating_request_settings_.reset();
   if (callback) {
     capture_interface_->Flush(std::move(callback));
   }