Handle ::content and :host-context correctly in SelectorQuery.

LayoutTests/fast/selectors/query-update-distribution-expected.txt

+Should update distribution when needed for querySelector and related methods.
+
+On success, you will see a series of "PASS" messages, followed by "TEST COMPLETE".
+
+
+PASS aRoot.querySelector(':host-context(#c) #d') is d
+PASS toArray(aRoot.querySelectorAll(':host-context(#c) #d')) is [d]
+PASS hostRoot.querySelector('::content #a') is null
+PASS toArray(hostRoot.querySelectorAll('::content #a')) is []
+PASS a.matches('::content #a') is false
+PASS d.matches(':host-context(#host) #d') is true
+PASS d.matches(':host-context(#c) #d') is true
+PASS b.closest('::content #a') is null
+PASS e.closest(':host-context(#host) #d') is d
+PASS successfullyParsed is true
+
+TEST COMPLETE
+