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 /content/browser/conversions

This CL was uploaded by git cl split.

R=csharrison@chromium.org

Bug: 1110266
Change-Id: I42bb363056e26229c924208094657ac22ddc3873
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2371288
Auto-Submit: Sean McAllister <smcallis@google.com>
Reviewed-by: Charlie Harrison <csharrison@chromium.org>
Commit-Queue: Charlie Harrison <csharrison@chromium.org>
Cr-Commit-Position: refs/heads/master@{#801131}

content/browser/conversions/conversion_manager_impl_unittest.cc

@@ -315,7 +315,7 @@ TEST_F(ConversionManagerImplTest, ConversionsSentFromUI_ReportedImmediately) {
 }
 
 // TODO(crbug.com/1088449): Flaky on Linux and Android.
-#if defined(OS_LINUX) || defined(OS_ANDROID)
+#if defined(OS_LINUX) || defined(OS_CHROMEOS) || defined(OS_ANDROID)
 #define MAYBE_ExpiredReportsAtStartup_Delayed \
   DISABLED_ExpiredReportsAtStartup_Delayed
 #else