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/command_buffer/common/gpu_memory_buffer_support.cc

This CL was uploaded by git cl split.

R=dcastagna@chromium.org

Bug: 1110266
Change-Id: Ib811894f612033723e634c7189a9b365aff353b6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2371295Reviewed-by: Daniele Castagna <dcastagna@chromium.org>
Commit-Queue: Sean McAllister <smcallis@google.com>
Cr-Commit-Position: refs/heads/master@{#801418}

gpu/command_buffer/common/gpu_memory_buffer_support.cc

@@ -55,7 +55,7 @@ bool IsImageSizeValidForGpuMemoryBufferFormat(const gfx::Size& size,
 uint32_t GetPlatformSpecificTextureTarget() {
 #if defined(OS_MAC)
   return macos_specific_texture_target;
-#elif defined(OS_ANDROID) || defined(OS_LINUX)
+#elif defined(OS_ANDROID) || defined(OS_LINUX) || defined(OS_CHROMEOS)
   return GL_TEXTURE_EXTERNAL_OES;
 #elif defined(OS_WIN) || defined(OS_FUCHSIA)
   return GL_TEXTURE_2D;
@@ -85,7 +85,7 @@ GPU_EXPORT uint32_t GetBufferTextureTarget(gfx::BufferUsage usage,
 
 GPU_EXPORT bool NativeBufferNeedsPlatformSpecificTextureTarget(
     gfx::BufferFormat format) {
-#if defined(USE_OZONE) || defined(OS_LINUX)
+#if defined(USE_OZONE) || defined(OS_LINUX) || defined(OS_CHROMEOS)
   // Always use GL_TEXTURE_2D as the target for RGB textures.
   // https://crbug.com/916728
   if (format == gfx::BufferFormat::R_8 || format == gfx::BufferFormat::RG_88 ||