Add support for reftests to conformance test harness

Currently the WebGL conformance test harness runs purely within a web page, which means that it can't take snapshots of the composited page; this is impossible on the web for security reasons.

With the WebDriver-based harness added by @gyagp in #2361, it is now conceptually possible to add reftests for WebGL. These would snapshot the result of compositing the WebGL canvas with the rest of the web page, and compare it against another visually equivalent web page. No golden images would be necessary.

This would aid in testing of features like the context creation attributes premultiplyAlpha and preserveDrawingBuffer, and in various compositing and dirtying scenarios. http://crbug.com/768969 is an example of a bug where the test could be written as a reftest, but not as a conformance test today.

Start by reading the existing conformance test harness and the WebDriver-based harness introduced in issue #2361. Determine how a reftest can capture the composited WebGL page and compare it with an equivalent page without golden images. Done means the harness can run these reftests for compositing-related cases such as premultiplyAlpha and preserveDrawingBuffer.

Written by the indexing model from the issue text.