[editing] Add SelectionModifier unit test for bug 1155342 and 1155309

Bug 1155342 was already fixed by r837219, but I didn't add a unit test
for SelectionModifier since the testcase that I had wasn't trivial.

However, turns out that bug 1155309 is a duplicate, but with a simpler
testcase which does seem a good fit for a unit test.

This new unit test would have crashed before r837219.

Bug: 1155342, 1155309
Change-Id: I52def532cf8eda92d6009e473d3a28ac4960cc2e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2596297Reviewed-by: Xiaocheng Hu <xiaochengh@chromium.org>
Commit-Queue: Oriol Brufau <obrufau@igalia.com>
Cr-Commit-Position: refs/heads/master@{#837905}

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

@@ -280,4 +280,16 @@ TEST_F(SelectionModifierTest, MoveCaretWithShadow) {
   }
 }
 
+// For https://crbug.com/1155342 and https://crbug.com/1155309
+TEST_F(SelectionModifierTest, PreviousParagraphOfObject) {
+  const SelectionInDOMTree selection =
+      SetSelectionTextToBody("<object>|</object>");
+  SelectionModifier modifier(GetFrame(), selection);
+  modifier.Modify(SelectionModifyAlteration::kMove,
+                  SelectionModifyDirection::kBackward,
+                  TextGranularity::kParagraph);
+  EXPECT_EQ("|<object></object>",
+            GetSelectionTextFromBody(modifier.Selection().AsSelection()));
+}
+
 }  // namespace blink