embedding: set string_list api functions to pub

fixes #3687
Make DOM getters that return &JS<T> return Temporary<T> instead #3707
auto merge of #3775 : zmike/servo/embedding-dead_code, r=jdm

fixes #3687 

r+ @jdm @larsbergstrom ?
auto merge of #3774 : ebalint/servo/3707_DOM_getters, r=jdm
Clear reflow flags after reflow.