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 /media/video/gpu_memory_buffer_video_frame_pool.cc

This CL was uploaded by git cl split.

R=dcastagna@chromium.org

Bug: 1110266
Change-Id: Ic8eb9551075af15901c576695ae94460e9fc3900
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2370731Reviewed-by: Daniele Castagna <dcastagna@chromium.org>
Commit-Queue: Sean McAllister <smcallis@google.com>
Cr-Commit-Position: refs/heads/master@{#801404}

media/video/gpu_memory_buffer_video_frame_pool.cc

@@ -573,7 +573,7 @@ void GpuMemoryBufferVideoFramePool::PoolImpl::CreateHardwareFrame(
   }
 
   bool is_software_backed_video_frame = !video_frame->HasTextures();
-#if defined(OS_LINUX)
+#if defined(OS_LINUX) || defined(OS_CHROMEOS)
   is_software_backed_video_frame &= !video_frame->HasDmaBufs();
 #endif