Disable SpeechRecognitionBrowserTest.Precheck on Mac as well.

I saw it flake even with reruns: http://build.chromium.org/p/tryserver.chromium.mac/builders/mac_chromium_rel_swarming/builds/881

BUG=396414
TBR=janx@chromium.org
NOTRY=true

Review URL: https://codereview.chromium.org/451173002

Cr-Commit-Position: refs/heads/master@{#288467}
git-svn-id: svn://svn.chromium.org/chrome/trunk/src@288467 0039d316-1c4b-4281-b951-d872f2087c98

content/browser/speech/speech_recognition_browsertest.cc

@@ -189,7 +189,7 @@ class SpeechRecognitionBrowserTest :
 // Simply loads the test page and checks if it was able to create a Speech
 // Recognition object in JavaScript, to make sure the Web Speech API is enabled.
 // http://crbug.com/396414
-#if defined(OS_WIN)
+#if defined(OS_WIN) || defined(OS_MACOSX)
 #define MAYBE_Precheck DISABLED_Precheck
 #else
 #define MAYBE_Precheck Precheck