Fix WebXR renderer crash

Adds early returns to XRWebGLDrawingBuffer if the drawing buffer client
is null. This fixes a segfault that was causing a renderer crash when
exiting (or entering?) WebXR presentation.

Bug: 833620
Cq-Include-Trybots: master.tryserver.blink:linux_trusty_blink_rel;master.tryserver.chromium.linux:linux_layout_tests_slimming_paint_v2
Change-Id: Iaf03bff95f542da63e871b1abf8a96e1b0b29d7f
Reviewed-on: https://chromium-review.googlesource.com/1014480Reviewed-by: Ian Vollick <vollick@chromium.org>
Commit-Queue: Brian Sheedy <bsheedy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#551361}

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

@@ -250,6 +250,8 @@ void XRWebGLDrawingBuffer::UseSharedBuffer(
   }
 
   DrawingBuffer::Client* client = drawing_buffer_->client();
+  if (!client)
+    return;
   client->DrawingBufferClientRestoreFramebufferBinding();
 }
 
@@ -284,6 +286,8 @@ void XRWebGLDrawingBuffer::DoneWithSharedBuffer() {
   shared_buffer_texture_id_ = 0;
 
   DrawingBuffer::Client* client = drawing_buffer_->client();
+  if (!client)
+    return;
   client->DrawingBufferClientRestoreFramebufferBinding();
 }