Fix an assertion failure in FrameSelection::selectAll().

Don't try to make VisibleSelection for elements of which content is not
selectable.

This CL has no user-visible behavior changes because
selectionFromContentsOfNode() created a null VisibleSelection before this CL.

BUG=477839
TEST=automated

Review URL: https://codereview.chromium.org/1647833002

Cr-Commit-Position: refs/heads/master@{#372015}

third_party/WebKit/Source/core/editing/FrameSelection.cpp

@@ -842,7 +842,7 @@ void FrameSelection::selectAll()
             selectStartTarget = document->body();
         }
     }
-    if (!root)
+    if (!root || editingIgnoresContent(root))
         return;
 
     if (selectStartTarget && !selectStartTarget->dispatchEvent(Event::createCancelableBubble(EventTypeNames::selectstart)))

third_party/WebKit/Source/core/editing/FrameSelectionTest.cpp

@@ -214,4 +214,12 @@ TEST_F(FrameSelectionTest, setNonDirectionalSelectionIfNeeded)
     EXPECT_EQ(PositionInComposedTree(bottom, 3), visibleSelectionInComposedTree().end());
 }
 
+TEST_F(FrameSelectionTest, SelectAllWithUnselectableRoot)
+{
+    RefPtrWillBeRawPtr<Element> select = document().createElement("select", ASSERT_NO_EXCEPTION);
+    document().replaceChild(select.get(), document().documentElement());
+    selection().selectAll();
+    EXPECT_TRUE(selection().isNone()) << "Nothing should be selected if the content of the documentElement is not selctable.";
+}
+
 } // namespace blink