Add comment for BLASTBufferQueue buffer barriers

    ui::Size mSize GUARDED_BY(mMutex);

    ui::Size mRequestedSize GUARDED_BY(mMutex);

    int32_t mFormat GUARDED_BY(mMutex);

    // Buffers may reach SurfaceFlinger out of order. Buffer barriers are used to ensure that

    // a buffer with a given frame number is only applied in SurfaceFlinger once a previous buffer

    // with a lower frame number is applied. Transactions that are not applied by BLASTBufferQueue

    // always wait on the last buffer applied by BLASTBufferQueue using these barriers.

    // Barriers are set on a layer, and are not shared across layers. This means if the layer

    // backing this BLASTBufferQueue changes, the barrier will be invalid. This flag is used to

    // track when the layer is changed so the barrier can be skipped.

    bool mSetBufferBarrier GUARDED_BY(mMutex) = false;

    // Keep a copy of the current picture profile handle, so it can be moved to a new