Mark fast/text/decorations-transformed.html as flaky on win

Mark the fast/text/decorations-transformed.html test as flaky on
Windows. Possibly caused by the switch to DW.

BUG=378610
TBR=pdr@chromium.org

Review URL: https://codereview.chromium.org/304183004

git-svn-id: svn://svn.chromium.org/blink/trunk@175099 bbb929c8-8fbe-4397-9dbb-9b2b20218538