layout: Support more types of selectors for style sharing.

This helps avoid problems with style sharing in common cases, often
caused by the user agent stylesheet.
Cleanup Element's AttributeHandlers

Remove notify_attribute_{changed,removed} in favor of a unified
notify_content_changed.
auto merge of #3859 : brunoabinader/servo/element-notify-content-changed, r=Ms2ger

Remove ```notify_attribute_{changed,removed}``` in favor of a unified ```notify_content_changed```. In the future, whether we decide to specialize ```document.content_changed()```, it'll be just a matter of adding a param.
Update azure. This removes the final rust-opengles dependency.
auto merge of #3861 : glennw/servo/update-azure, r=metajack
auto merge of #3838 : pcwalton/servo/style-sharing-enhancements, r=SimonSapin

This helps avoid problems with style sharing in common cases, often
caused by the user agent stylesheet.

r? @SimonSapin