Disabled accessibility browser test for content editable descendents.

Required in order to reland CL 1056763002. The only difference in the test output is that on the Mac the textual contents of the contenteditable are also present in the AXValue property on the AXGroup representing the contenteditable. I prefer to reland the contenteditable CL in order to investigate this more easily on my Mac.
BUG=347859
R=dmazzoni

Review URL: https://codereview.chromium.org/1080533004

Cr-Commit-Position: refs/heads/master@{#325452}

content/browser/accessibility/dump_accessibility_tree_browsertest.cc

@@ -686,7 +686,7 @@ IN_PROC_BROWSER_TEST_F(DumpAccessibilityTreeTest, AccessibilityDt) {
     AccessibilityContenteditableDescendants
 #endif
 IN_PROC_BROWSER_TEST_F(DumpAccessibilityTreeTest,
-                       MAYBE_AccessibilityContenteditableDescendants) {
+                       DISABLED_AccessibilityContenteditableDescendants) {
   RunHtmlTest(FILE_PATH_LITERAL("contenteditable-descendants.html"));
 }