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 /ui/aura

This CL was uploaded by git cl split.

R=sky@chromium.org

Bug: 1110266
Change-Id: I96cd101fb7cc5daf4e2398c9a4b7e80448c623a3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2371303
Auto-Submit: Sean McAllister <smcallis@google.com>
Reviewed-by: Scott Violet <sky@chromium.org>
Commit-Queue: Scott Violet <sky@chromium.org>
Cr-Commit-Position: refs/heads/master@{#801017}

ui/aura/test/ui_controls_ozone.cc

@@ -348,7 +348,7 @@ bool UIControlsOzone::ScreenDIPToHostPixels(gfx::PointF* location,
 // To avoid multiple definitions when use_x11 && use_ozone is true, disable this
 // factory method for OS_LINUX as Linux has a factory method that decides what
 // UIControls to use based on IsUsingOzonePlatform feature flag.
-#if !defined(OS_LINUX)
+#if !defined(OS_LINUX) && !defined(OS_CHROMEOS)
 ui_controls::UIControlsAura* CreateUIControlsAura(WindowTreeHost* host) {
   return new UIControlsOzone(host);
 }