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 /chrome/browser/web_applications/components/web_app_file_handler_registration.h

This CL was uploaded by git cl split.

R=davidbienvenu@chromium.org

Bug: 1110266
Change-Id: I3f6ea288380e5031072ac0d38288a6f44cffc389
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2370285
Auto-Submit: Sean McAllister <smcallis@google.com>
Reviewed-by: David Bienvenu <davidbienvenu@chromium.org>
Commit-Queue: David Bienvenu <davidbienvenu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#800901}

chrome/browser/web_applications/components/web_app_file_handler_registration.h

@@ -38,7 +38,7 @@ void RegisterFileHandlersWithOs(const AppId& app_id,
 // If a shim app was required, also removes the shim app.
 void UnregisterFileHandlersWithOs(const AppId& app_id, Profile* profile);
 
-#if defined(OS_LINUX)
+#if defined(OS_LINUX) || defined(OS_CHROMEOS)
 using RegisterMimeTypesOnLinuxCallback =
     base::OnceCallback<bool(base::FilePath profile_path,
                             std::string file_contents)>;