Re-mark hidpi canvas tests as skipped in test expectations

These tests turned out to be flaky [1] so I'm re-marking them as skip.

[1] http://test-results.appspot.com/dashboards/flakiness_dashboard.html#group=%40ToT%20Blink&tests=fast%2Fhidpi%2Fimage-srcset-relative-svg-canvas.html%2Cfast%2Fhidpi%2Fimage-srcset-relative-svg-canvas-2x.html

BUG=306222
TBR=yoav, hclam
NOTRY=true

Review URL: https://codereview.chromium.org/303443008

git-svn-id: svn://svn.chromium.org/blink/trunk@174824 bbb929c8-8fbe-4397-9dbb-9b2b20218538