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 /services/device/usb

This CL was uploaded by git cl split.

R=odejesush@chromium.org

Bug: 1110266
Change-Id: I368ba7aa63c20ff200577a3ba8bd8441d2cef1c1
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2371300Reviewed-by: Ovidio de Jesús Ruiz-Henríquez <odejesush@chromium.org>
Commit-Queue: Sean McAllister <smcallis@google.com>
Cr-Commit-Position: refs/heads/master@{#801636}

services/device/usb/usb_context_unittest.cc

@@ -28,7 +28,7 @@ class UsbContextTest : public testing::Test {
 
 }  // namespace
 
-#if defined(OS_LINUX)
+#if defined(OS_LINUX) || defined(OS_CHROMEOS)
 // Linux trybot does not support usb.
 #define MAYBE_GracefulShutdown DISABLED_GracefulShutdown
 #elif defined(OS_ANDROID)