gpu_init: Stop inlining GpuInit::TakeDefaultOffscreenSurface()

This fixes the GCC build after commit b6ef3ba ("Pass GpuPreferences to
in-process GPU in Mash"):

    In file included from ../../content/utility/utility_service_factory.h:12,
                     from ../../content/utility/utility_service_factory.cc:5:
    ../../base/memory/scoped_refptr.h: In instantiation of ‘static void scoped_refptr<T>::Release(T*) [with T = gl::GLSurface]’:
    ../../base/memory/scoped_refptr.h:208:14:   required from ‘scoped_refptr<T>::~scoped_refptr() [with T = gl::GLSurface]’
    ../../gpu/ipc/service/gpu_init.h:70:48:   required from here
    ../../base/memory/scoped_refptr.h:284:8: error: invalid use of incomplete type ‘class gl::GLSurface’
       ptr->Release();
       ~~~~~^~~~~~~
    In file included from ../../services/viz/service.h:8,
                     from ../../content/utility/utility_service_factory.cc:35:
    ../../gpu/ipc/service/gpu_init.h:22:7: note: forward declaration of ‘class gl::GLSurface’
     class GLSurface;
           ^~~~~~~~~

gpu_init.h forward-declares gl::GLSurface but using std::move() with GCC
seems to require a full declaration for things to work.

Bug: 819294
Change-Id: Ib22ca2f8af6977869571292acf780042507a8afd
Reviewed-on: https://chromium-review.googlesource.com/c/1365249
Reviewed-by: Jonathan Backer <backer@chromium.org>
Commit-Queue: Raphael Kubo da Costa <raphael.kubo.da.costa@intel.com>
Cr-Commit-Position: refs/heads/master@{#614358}

gpu/ipc/service/gpu_init.cc

@@ -519,4 +519,8 @@ void GpuInit::AdjustInfoToSwiftShader() {
   CollectContextGraphicsInfo(&gpu_info_, gpu_preferences_);
 }
 
+scoped_refptr<gl::GLSurface> GpuInit::TakeDefaultOffscreenSurface() {
+  return std::move(default_offscreen_surface_);
+}
+
 }  // namespace gpu

gpu/ipc/service/gpu_init.h

@@ -66,9 +66,7 @@ class GPU_IPC_SERVICE_EXPORT GpuInit {
   std::unique_ptr<GpuWatchdogThread> TakeWatchdogThread() {
     return std::move(watchdog_thread_);
   }
-  scoped_refptr<gl::GLSurface> TakeDefaultOffscreenSurface() {
-    return std::move(default_offscreen_surface_);
-  }
+  scoped_refptr<gl::GLSurface> TakeDefaultOffscreenSurface();
   bool init_successful() const { return init_successful_; }
 #if BUILDFLAG(ENABLE_VULKAN)
   VulkanImplementation* vulkan_implementation() {