Update from chromium https://crrev.com/301315

gpu/command_buffer/service/gpu_switches.cc

Index: gpu/command_buffer/service/gpu_switches.cc
diff --git a/gpu/command_buffer/service/gpu_switches.cc b/gpu/command_buffer/service/gpu_switches.cc
index 0491c413e5e3d7570fd3c9721c5bb72fb4fc8e94..8f5bc35cf407344b282aeb778e73be2b48357272 100644
--- a/gpu/command_buffer/service/gpu_switches.cc
+++ b/gpu/command_buffer/service/gpu_switches.cc
@@ -56,6 +56,11 @@ const char kDisableGpuShaderDiskCache[]     = "disable-gpu-shader-disk-cache";
 const char kEnableShareGroupAsyncTextureUpload[] =
     "enable-share-group-async-texture-upload";
 
+// Simulates shared textures when share groups are not available. Not available
+// everywhere.
+const char kEnableThreadedTextureMailboxes[] =
+    "enable-threaded-texture-mailboxes";
+
 const char* kGpuSwitches[] = {
   kCompileShaderAlwaysSucceeds,
   kDisableGLErrorLimit,