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 /printing

This CL was uploaded by git cl split.

R=weili@chromium.org

Bug: 1110266
Change-Id: I1967dae5a54195670fb618ee9f0758bbd2fd28db
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2370732
Auto-Submit: Sean McAllister <smcallis@google.com>
Reviewed-by: Wei Li <weili@chromium.org>
Commit-Queue: Wei Li <weili@chromium.org>
Cr-Commit-Position: refs/heads/master@{#801099}

printing/print_settings_conversion.cc

@@ -180,7 +180,7 @@ bool PrintSettingsFromJobSettings(const base::Value& job_settings,
       !pages_per_sheet.has_value()) {
     return false;
   }
-#if defined(OS_WIN) || defined(OS_LINUX)
+#if defined(OS_WIN) || defined(OS_LINUX) || defined(OS_CHROMEOS)
   base::Optional<int> dpi_horizontal =
       job_settings.FindIntKey(kSettingDpiHorizontal);
   base::Optional<int> dpi_vertical =