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

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

Index: LayoutTests/fast/selectors/query-update-distribution-expected.txt
diff --git a/LayoutTests/fast/selectors/query-update-distribution-expected.txt b/LayoutTests/fast/selectors/query-update-distribution-expected.txt
new file mode 100644
index 0000000000000000000000000000000000000000..3cc109b64ffa27e1c14e574d5800323aab25d358
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+++ b/LayoutTests/fast/selectors/query-update-distribution-expected.txt
@@ -0,0 +1,15 @@
+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 targetRoot.querySelector('::content #a') is null
+PASS a.matches('::content #a') is false
+PASS d.matches(':host-context(#target) #d') is true
+PASS d.matches(':host-context(#c) #d') is true
+PASS b.closest('::content #a') is null
+PASS successfullyParsed is true
+
+TEST COMPLETE
+