Refactor OS_LINUX preprocessor directive for LaCrOS effort.

Currently, ChromeOS defines the OS_LINUX directive as well as
OS_CHROMEOS.  We're working to separate these two, so we're
making the fact that OS_LINUX == OS_LINUX || OS_CHROMEOS
explicit.

This is changes for /gpu/ipc/service/gpu_memory_buffer_factory.cc

This CL was uploaded by git cl split.

R=dcastagna@chromium.org

Bug: 1110266
Change-Id: Ie1146726ff4301d6373d5bb777a9bde87b00954f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2371298Reviewed-by: Daniele Castagna <dcastagna@chromium.org>
Commit-Queue: Sean McAllister <smcallis@google.com>
Cr-Commit-Position: refs/heads/master@{#801420}

gpu/ipc/service/gpu_memory_buffer_factory.cc

@@ -12,7 +12,7 @@
 #include "gpu/ipc/service/gpu_memory_buffer_factory_io_surface.h"
 #endif
 
-#if defined(OS_LINUX) || defined(OS_FUCHSIA)
+#if defined(OS_LINUX) || defined(OS_CHROMEOS) || defined(OS_FUCHSIA)
 #include "gpu/ipc/service/gpu_memory_buffer_factory_native_pixmap.h"
 #endif
 
@@ -34,7 +34,7 @@ GpuMemoryBufferFactory::CreateNativeType(
   return std::make_unique<GpuMemoryBufferFactoryIOSurface>();
 #elif defined(OS_ANDROID)
   return std::make_unique<GpuMemoryBufferFactoryAndroidHardwareBuffer>();
-#elif defined(OS_LINUX) || defined(OS_FUCHSIA)
+#elif defined(OS_LINUX) || defined(OS_CHROMEOS) || defined(OS_FUCHSIA)
   return std::make_unique<GpuMemoryBufferFactoryNativePixmap>(
       vulkan_context_provider);
 #elif defined(OS_WIN)