Bug 404077:  Do GC in a small number of tests to reduce the spread of GC-related assertions.  r=Jesse
Bug 404077:  Clean up erroneous expectAssertions calls from GC of objects created in test_bug391777.html.
Bug 404077:  Clean up erroneous expectAssertions calls from GC of objects created in test_bug291653.html.
Bug 404077:  Clean up erroneous expectAssertions calls from GC of objects created in a series of tests using showModalDialog.

This will require a little follow-up cleanup.
Bug 404077:  Use navigator.platform.startsWith rather than navigator.platform.indexOf, at Jesse's suggestion.
Bug 404077:  Annotate one more known assertion.