Mark image-currentsrc.html as being flaky.

The test is still very flaky on Windows, and somewhat flaky on other
platforms.

TBR=jchaffraix@chromium.org, yoav@yoav.ws
BUG=380664

Review URL: https://codereview.chromium.org/311133002

git-svn-id: svn://svn.chromium.org/blink/trunk@175483 bbb929c8-8fbe-4397-9dbb-9b2b20218538