Disable MouseEventsTest.ModalDialog on Linux

The test fails very often, and sometimes times out. See bug.

No-try: true
No-presubmit: true
Bug: 838120
Change-Id: Ibdc63e23418b47757b53f079bb2841f35d9cf65c
Reviewed-on: https://chromium-review.googlesource.com/1034802
Commit-Queue: Christos Froussios <cfroussios@chromium.org>
Reviewed-by: Christos Froussios <cfroussios@chromium.org>
Cr-Commit-Position: refs/heads/master@{#554734}

chrome/browser/mouse_events_interactive_uitest.cc

@@ -165,12 +165,13 @@ IN_PROC_BROWSER_TEST_F(MouseEventsTest, MAYBE_ContextMenu) {
   EXPECT_EQ(success_title, done_title_watcher.WaitAndGetTitle());
 }
 
-#if defined(OS_WIN) || defined(OS_MACOSX)
+#if defined(OS_WIN) || defined(OS_MACOSX) || defined(OS_LINUX)
 // Test that a mouseleave is not triggered when showing a modal dialog.
 // Sample regression: crbug.com/394672
 // TODO: Make test pass on OS_WIN and OS_MACOSX
 // OS_WIN: http://crbug.com/450138
 // OS_MACOSX: Missing automation provider support: http://crbug.com/45892.
+// OS_LINUX: Flaky http://crbug.com/838120
 #define MAYBE_ModalDialog DISABLED_ModalDialog
 #else
 #define MAYBE_ModalDialog ModalDialog