Don't fail on invalid JS syntax in event handlers.

Fixes #2862.
Merge pull request #2863 from mbrubeck/2862-null-fun

Don't fail on invalid JS syntax in event handlers. r=Ms2ger
Update to reflect rust-layers changes

It is no longer necessary to flush pending buffer requests when adding
buffers nor to manually create textures for layers.
Merge pull request #2870 from mrobinson/more-cleanup

Update to reflect rust-layers changes
Add rust-url as a submodule and a Makefile for it.
Start dogfooding rust-url. Fix #1673.