Workaround to avoid crashing in FrameSelection::SelectedText()

This patch introduces workaround to check invalid selection for avoiding crash
in |ComputeRangeForSerialization()| called by |FrameSelection::SelectedText()|.

When selection is none, both selection base and extent should be null. However,
we have invalid selection, one of base and extent is null, by some reasons.


Background: 
Bug: 1019152
Change-Id: I38717ede505eebb9c59229d5d2e14d75b50f24a5
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1891819
Commit-Queue: Yoshifumi Inoue <yosin@chromium.org>
Commit-Queue: Koji Ishii <kojii@chromium.org>
Reviewed-by: Koji Ishii <kojii@chromium.org>
Auto-Submit: Yoshifumi Inoue <yosin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#711162}

third_party/blink/renderer/core/editing/frame_selection.cc

@@ -920,9 +920,17 @@ void FrameSelection::SetFocusedNodeIfNeeded() {
 }
 
 static EphemeralRangeInFlatTree ComputeRangeForSerialization(
-    const SelectionInDOMTree& selection) {
-  const EphemeralRangeInFlatTree& range =
-      ConvertToSelectionInFlatTree(selection).ComputeRange();
+    const SelectionInDOMTree& selection_in_dom_tree) {
+  const SelectionInFlatTree& selection =
+      ConvertToSelectionInFlatTree(selection_in_dom_tree);
+  // TODO(crbug.com/1019152): Once we know the root cause of having
+  // seleciton with |base.IsNull() != extent.IsNull()|, we should get rid of
+  // this if-statement.
+  if (selection.Base().IsNull() || selection.Extent().IsNull()) {
+    DCHECK(selection.IsNone());
+    return EphemeralRangeInFlatTree();
+  }
+  const EphemeralRangeInFlatTree& range = selection.ComputeRange();
   const PositionInFlatTree& start =
       CreateVisiblePosition(range.StartPosition()).DeepEquivalent();
   const PositionInFlatTree& end =