Mark inspector/profiler/heap-snapshot-loader.html as flaky

The test fails time to time after heap profiler tests have switched from fake worker to real one.

BUG=330617
TBR=loislo@chromium.org

Review URL: https://codereview.chromium.org/141613002

git-svn-id: svn://svn.chromium.org/blink/trunk@165285 bbb929c8-8fbe-4397-9dbb-9b2b20218538