Mark svg text tests as flaky on mac

Turs out the svg text tests are flaky on Mac and not different
across Release/Debug. Likely due to test ordering and the font
cache. Marking as flaky while investegating.

TBR=eseidel@chromium.org
BUG=

Review URL: https://codereview.chromium.org/97493002

git-svn-id: svn://svn.chromium.org/blink/trunk@162929 bbb929c8-8fbe-4397-9dbb-9b2b20218538