Avoid WebXR resource leak on destruction

The DrawingBuffer code that WebXRDrawingBuffer is based on requires
a BeginDestruction method to be called to break a reference loop.
Both DrawingBuffer and its ColorBuffer subclass are refcounted and
store references to each other.

Add the BeginDestruction method to WebXRDrawingBuffer which was mentioned
in a copied comment but not implemented.

BUG=857547

Cq-Include-Trybots: luci.chromium.try:linux_layout_tests_slimming_paint_v2;master.tryserver.blink:linux_trusty_blink_rel
Change-Id: I3bef8e975c26ca432eafe7eb4ba8c4daae715796
Reviewed-on: https://chromium-review.googlesource.com/1120645Reviewed-by: Ian Vollick <vollick@chromium.org>
Commit-Queue: Klaus Weidner <klausw@chromium.org>
Cr-Commit-Position: refs/heads/master@{#571693}

third_party/blink/renderer/modules/xr/xr_webgl_layer.cc

@@ -121,10 +121,10 @@ XRWebGLLayer::XRWebGLLayer(XRSession* session,
                            double framebuffer_scale)
     : XRLayer(session, kXRWebGLLayerType),
       webgl_context_(webgl_context),
-      drawing_buffer_(drawing_buffer),
+      drawing_buffer_(std::move(drawing_buffer)),
       framebuffer_(framebuffer),
       framebuffer_scale_(framebuffer_scale) {
-  DCHECK(drawing_buffer);
+  DCHECK(drawing_buffer_);
   // If the contents need mirroring, indicate that to the drawing buffer.
   if (session->exclusive() && session->outputContext() &&
       session->device()->external()) {
@@ -137,6 +137,7 @@ XRWebGLLayer::XRWebGLLayer(XRSession* session,
 XRWebGLLayer::~XRWebGLLayer() {
   if (mirroring_)
     drawing_buffer_->SetMirrorClient(nullptr);
+  drawing_buffer_->BeginDestruction();
 }
 
 void XRWebGLLayer::getXRWebGLRenderingContext(

third_party/blink/renderer/platform/graphics/gpu/xr_webgl_drawing_buffer.cc

@@ -114,6 +114,12 @@ XRWebGLDrawingBuffer::XRWebGLDrawingBuffer(DrawingBuffer* drawing_buffer,
       alpha_(want_alpha_channel),
       multiview_(false) {}
 
+void XRWebGLDrawingBuffer::BeginDestruction() {
+  back_color_buffer_ = nullptr;
+  front_color_buffer_ = nullptr;
+  recycled_color_buffer_queue_.clear();
+}
+
 // TODO(bajones): The GL resources allocated in this function are leaking. Add
 // a way to clean up the buffers when the layer is GCed or the session ends.
 bool XRWebGLDrawingBuffer::Initialize(const IntSize& size,

third_party/blink/renderer/platform/graphics/gpu/xr_webgl_drawing_buffer.h

@@ -63,13 +63,18 @@ class PLATFORM_EXPORT XRWebGLDrawingBuffer
   void UseSharedBuffer(const gpu::MailboxHolder&);
   void DoneWithSharedBuffer();
 
+  // Prepare for destruction by breaking reference loops. This must be called to
+  // avoid memory leaks, drawing buffer and color buffers are refcounted and
+  // store references to each other.
+  void BeginDestruction();
+
  private:
   struct ColorBuffer : public RefCounted<ColorBuffer> {
     ColorBuffer(XRWebGLDrawingBuffer*, const IntSize&, GLuint texture_id);
     ~ColorBuffer();
 
     // The owning XRWebGLDrawingBuffer. Note that DrawingBuffer is explicitly
-    // destroyed by the beginDestruction method, which will eventually drain all
+    // destroyed by the BeginDestruction method, which will eventually drain all
     // of its ColorBuffers.
     scoped_refptr<XRWebGLDrawingBuffer> drawing_buffer;
     const IntSize size;