Renamed gc() function in js-test-pre to allow Chrome's native implementation to be exposed.

Added conformance/context/user-defined-properties-on-context.html test to ensure that user-defined properties on the WebGL context itself aren't lost after garbage collection. This test currently passes in Firefox and Safari and fails in Chrome.
Added new test with --min-version=1.0.4.
Merge pull request #800 from kenrussell/more-stringent-gc

Renamed gc() function in js-test-pre to allow Chrome's native implementa...