Avoid loading DRI via GBM when GpuMemoryBuffers are disabled

We haven't yet whitelisted the necessary dri files in the GPU sandbox,
which is leading to issues like 1077609 and 1077626.  Since GMBs are
not yet supported, avoid loading GBM unless
--enable-native-gpu-memory-buffers is passed.

Bug: 1077609, 1077626, 1031269
Change-Id: Ic052d2e89330c6558da86a91b77637229808102f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2211120
Auto-Submit: Thomas Anderson <thomasanderson@chromium.org>
Reviewed-by: Kenneth Russell <kbr@chromium.org>
Commit-Queue: Thomas Anderson <thomasanderson@chromium.org>
Cr-Commit-Position: refs/heads/master@{#770878}

content/gpu/gpu_main.cc

@@ -364,11 +364,13 @@ int GpuMain(const MainFunctionParams& parameters) {
 #if defined(USE_X11)
   // ui::GbmDevice() takes >50ms with amdgpu, so kick off
   // GpuMemoryBufferSupportX11 creation on another thread now.
-  base::PostTask(
-      FROM_HERE, base::BindOnce([]() {
-        SCOPED_UMA_HISTOGRAM_TIMER("Linux.X11.GbmSupportX11CreationTime");
-        ui::GpuMemoryBufferSupportX11::GetInstance();
-      }));
+  if (gpu_preferences.enable_native_gpu_memory_buffers) {
+    base::PostTask(
+        FROM_HERE, base::BindOnce([]() {
+          SCOPED_UMA_HISTOGRAM_TIMER("Linux.X11.GbmSupportX11CreationTime");
+          ui::GpuMemoryBufferSupportX11::GetInstance();
+        }));
+  }
 #endif
 
   auto* client = GetContentClient()->gpu();