Always use a UI message loop in the GPU process on Windows.

ANGLE can now create child windows and requires UI messages to be handled.
Because it is not possible to determine if ANGLE will use a child window
when creating surfaces, always use a UI message loop.

BUG=angleproject:961

Review URL: https://codereview.chromium.org/1097283005

Cr-Commit-Position: refs/heads/master@{#327140}

content/gpu/gpu_main.cc

@@ -155,16 +155,9 @@ int GpuMain(const MainFunctionParams& parameters) {
   bool dead_on_arrival = false;
 
 #if defined(OS_WIN)
-  base::MessageLoop::Type message_loop_type = base::MessageLoop::TYPE_IO;
-  // Unless we're running on desktop GL, we don't need a UI message
-  // loop, so avoid its use to work around apparent problems with some
-  // third-party software.
-  if (command_line.HasSwitch(switches::kUseGL) &&
-      command_line.GetSwitchValueASCII(switches::kUseGL) ==
-          gfx::kGLImplementationDesktopName) {
-    message_loop_type = base::MessageLoop::TYPE_UI;
-  }
-  base::MessageLoop main_message_loop(message_loop_type);
+  // Use a UI message loop because ANGLE and the desktop GL platform can
+  // create child windows to render to.
+  base::MessageLoop main_message_loop(base::MessageLoop::TYPE_UI);
 #elif defined(OS_LINUX) && defined(USE_X11)
   // We need a UI loop so that we can grab the Expose events. See GLSurfaceGLX
   // and https://crbug.com/326995.