Disable flaky ExtensionInstallUIBrowserTest.TestInstallThemeInFullScreen

The test is flaky on platforms other than mac.

TBR=rdevlin.cronin@chromium.org

Bug: 851252
Change-Id: I72f099a12c5c99c52eedd6bde6c335f8f169b61a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1855064Reviewed-by: Hayato Ito <hayato@chromium.org>
Commit-Queue: Hayato Ito <hayato@chromium.org>
Cr-Commit-Position: refs/heads/master@{#705000}

chrome/browser/extensions/extension_install_ui_browsertest.cc

@@ -149,14 +149,9 @@ IN_PROC_BROWSER_TEST_F(ExtensionInstallUIBrowserTest,
   ASSERT_FALSE(GetTheme());
 }
 
-// Flaky on Mac (http://crbug.com/851252).
-#if defined(OS_MACOSX)
-#define MAYBE_TestInstallThemeInFullScreen DISABLED_TestInstallThemeInFullScreen
-#else
-#define MAYBE_TestInstallThemeInFullScreen TestInstallThemeInFullScreen
-#endif
+// Flaky (http://crbug.com/851252).
 IN_PROC_BROWSER_TEST_F(ExtensionInstallUIBrowserTest,
-                       MAYBE_TestInstallThemeInFullScreen) {
+                       DISABLED_TestInstallThemeInFullScreen) {
   EXPECT_TRUE(chrome::ExecuteCommand(browser(), IDC_FULLSCREEN));
   InstallThemeAndVerify("theme", "camo theme");
 }