Modify Linux travis config to run content and cpu ref tests.
Add support for showing debug borders

This can help debug issues with the compositor and show when a page is
creating layers.
Merge pull request #2912 from mrobinson/debug-borders

Add support for showing debug borders
Merge pull request #2904 from glennw/linux-ref-test

Modify Linux travis config to run content and cpu ref tests.
Implement very basic canvas rendering context logic. No visual display present.