Change TestRendererAccessibilityTree from disabled to flaky.
Change TestRendererAccessibilityTree from disabled to flaky.

Check that document_accessible is non null so that the test doesn't crash.

BUG=44546
TEST=NONE

Review URL: http://codereview.chromium.org/2070017

git-svn-id: http://src.chromium.org/svn/trunk/src@48084 4ff67af0-8c30-449e-8e8b-ad334ec8d88c

Former-commit-id: b9f4a380b74a25a0c56b41885d14c12fa3da3086
meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser