[Origin Trials] Add tests for static methods on partial interfaces

This adds bindings an layout tests for static methods defined on partial
interfaces under an origin trial, which were missing from the test suite.

BUG=614352,621641
R=chasej@chromium.org

Review-Url: https://codereview.chromium.org/2181233004
Cr-Commit-Position: refs/heads/master@{#407903}
5 files changed