Introduce Position::inDocument()

Source/core/editing/ApplyBlockElementCommand.cpp

Index: Source/core/editing/ApplyBlockElementCommand.cpp
diff --git a/Source/core/editing/ApplyBlockElementCommand.cpp b/Source/core/editing/ApplyBlockElementCommand.cpp
index 7fc6e6d43a592f9562e792140c9d420f85574d9a..938ee5b9abc1f9acbd1163f0e59c3695a3870cdc 100644
--- a/Source/core/editing/ApplyBlockElementCommand.cpp
+++ b/Source/core/editing/ApplyBlockElementCommand.cpp
@@ -143,11 +143,11 @@ void ApplyBlockElementCommand::formatSelection(const VisiblePosition& startOfSel
         // indentIntoBlockquote could move more than one paragraph if the paragraph
         // is in a list item or a table. As a result, endAfterSelection could refer to a position
         // no longer in the document.
-        if (endAfterSelection.isNotNull() && !endAfterSelection.deepEquivalent().anchorNode()->inDocument())
+        if (endAfterSelection.isNotNull() && !endAfterSelection.deepEquivalent().inDocument())
             break;
         // Sanity check: Make sure our moveParagraph calls didn't remove endOfNextParagraph.deepEquivalent().deprecatedNode()
         // If somehow we did, return to prevent crashes.
-        if (endOfNextParagraph.isNotNull() && !endOfNextParagraph.deepEquivalent().anchorNode()->inDocument()) {
+        if (endOfNextParagraph.isNotNull() && !endOfNextParagraph.deepEquivalent().inDocument()) {
             ASSERT_NOT_REACHED();
             return;
         }