Make StartOfSentencePosition() to return null for null-position

This patch changes |StartOfSentencePosition()| to return null-position for
null-position to avoid to hit DcHECK for non-null position in
|TextSegments::FindBoundaryBackward()|.

Bug: 1100971
Change-Id: Iacaa706be454ef0334c1524703eaeddf56bfe1dd
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2280978
Commit-Queue: Yoshifumi Inoue <yosin@chromium.org>
Commit-Queue: Kent Tamura <tkent@chromium.org>
Reviewed-by: Kent Tamura <tkent@chromium.org>
Auto-Submit: Yoshifumi Inoue <yosin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#785331}

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

@@ -104,4 +104,16 @@ TEST_F(SelectionModifierTest, PreviousLineWithDisplayNone) {
       GetSelectionTextFromBody(modifier.Selection().AsSelection()));
 }
 
+// For http://crbug.com/1100971
+TEST_F(SelectionModifierTest, StartOfSentenceWithNull) {
+  InsertStyleElement("b {display:inline-block}");
+  const SelectionInDOMTree selection =
+      SetSelectionTextToBody("|<b><ruby><a></a></ruby></b>");
+  SelectionModifier modifier(GetFrame(), selection);
+  // We call |StartOfSentence()| with null-position.
+  EXPECT_FALSE(modifier.Modify(SelectionModifyAlteration::kMove,
+                               SelectionModifyDirection::kBackward,
+                               TextGranularity::kSentenceBoundary));
+}
+
 }  // namespace blink

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

@@ -288,6 +288,8 @@ VisiblePosition PreviousSentencePosition(const VisiblePosition& c) {
 // ----
 
 PositionInFlatTree StartOfSentencePosition(const PositionInFlatTree& position) {
+  if (position.IsNull())
+    return position;
   const PositionInFlatTree result = StartOfSentenceInternal(position);
   return AdjustBackwardPositionToAvoidCrossingEditingBoundaries(
              PositionInFlatTreeWithAffinity(result), position)