Unify if guards of GetLinuxWindowManager.

This comes from a review comment on http://crrev.com/2917483002. The
if guard on the function call to GetLinuxWindowManager is no longer
the same as the guard of the function definition.

This CL makes them the same again.

BUG=724244
R=thestig@chromium.org
CQ_INCLUDE_TRYBOTS=master.tryserver.chromium.linux:linux_chromium_ozone_compile_only_ng

Review-Url: https://codereview.chromium.org/2914963003
Cr-Commit-Position: refs/heads/master@{#476481}

chrome/browser/metrics/chrome_browser_main_extra_parts_metrics.cc

@@ -283,7 +283,7 @@ void RecordLinuxGlibcVersion() {
 #endif
 }
 
-#if defined(USE_X11) && !defined(OS_CHROMEOS)
+#if defined(OS_LINUX) && defined(USE_X11) && !defined(OS_CHROMEOS)
 UMALinuxWindowManager GetLinuxWindowManager() {
   switch (ui::GuessWindowManager()) {
     case ui::WM_OTHER: