Mark BrowserFocusTest.BackgroundBrowserDontStealFocus flaky on Linux

TBR=battre@chromium.org
NOTRY=true

Bug: 62538
Change-Id: I306085f0649628fbc0bdabfe4a58a83b29daa13d
Reviewed-on: https://chromium-review.googlesource.com/848077Reviewed-by: Xida Chen <xidachen@chromium.org>
Commit-Queue: Xida Chen <xidachen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#526698}

chrome/browser/ui/browser_focus_uitest.cc

@@ -360,7 +360,7 @@ IN_PROC_BROWSER_TEST_F(BrowserFocusTest, TabsRememberFocusFindInPage) {
 
 // Background window does not steal focus.
 // Flaky, http://crbug.com/62538.
-#if defined(OS_CHROMEOS)
+#if defined(OS_CHROMEOS) || defined(OS_LINUX)
 #define MAYBE_BackgroundBrowserDontStealFocus \
   DISABLED_BackgroundBrowserDontStealFocus
 #else