Disable WebUIMojoTest.EndToEndPing on Linux

BUG=418019
TEST=linux bots, content_browsertests
TBR=viettrungluu@chromium.org
NOTRY=true

Review URL: https://codereview.chromium.org/612483002

Cr-Commit-Position: refs/heads/master@{#296979}

content/browser/webui/web_ui_mojo_browsertest.cc

@@ -197,8 +197,8 @@ class WebUIMojoTest : public ContentBrowserTest {
 
 // Loads a webui page that contains mojo bindings and verifies a message makes
 // it from the browser to the page and back.
-// Fails on Win only.  http://crbug.com/418019
-#if defined(OS_WIN)
+// Fails on Win and Linux.  http://crbug.com/418019
+#if defined(OS_WIN) || defined(OS_LINUX)
 #define MAYBE_EndToEndPing DISABLED_EndToEndPing
 #else
 #define MAYBE_EndToEndPing EndToEndPing