Don't use the BufferManager if it doesn't exist in the GPU fuzzer.

TBR=zmo@chromium.org

BUG=1056984
BUG=1030835

Change-Id: I491b8c1d38f9637d37a92cb6d4b84e7ff2a5d7e4
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2079463Reviewed-by: Geoff Lang <geofflang@chromium.org>
Commit-Queue: Geoff Lang <geofflang@chromium.org>
Cr-Commit-Position: refs/heads/master@{#745501}

gpu/command_buffer/tests/fuzzer_main.cc

@@ -456,7 +456,9 @@ class CommandBufferSetup {
 
     decoder_->set_max_bucket_size(8 << 20);
 #if !defined(GPU_FUZZER_USE_RASTER_DECODER)
-    context_group->buffer_manager()->set_max_buffer_size(8 << 20);
+    if (context_group->buffer_manager()) {
+        context_group->buffer_manager()->set_max_buffer_size(8 << 20);
+    }
 #endif
     return decoder_->MakeCurrent();
   }